7. Word Problems Flashcards
What are the four steps to breaking down any word problem?
(1) Desired. Identify the desired value (i.e. what the question is asking for)
(2) Unknowns. Identify unknown values and label them with variables (never forget units!)
(3) Relationships. Identify relationships and translate them into equations
(4) Solve. Use the equations to solve for the desired value
* NOTE: Generally, the most efficient way to find the desired value is to eliminate unwanted variables using substitution
* Steps do not need to be followed in strict order
What rule applies to additive relationships?
The units of every term must be the same! Adding terms with the same units does not change the units
What rule applies to multiplicative relationships?
Treat units like numerators and denominators. Units that are multiplied together DO change!
What are some common word problem phrases for addition?
- Add, sum, total (of parts), more than: +
- The sum of x and y: x + y
- The sum of the three funds combined: a + b + c
- When fifty is added to his age: a + 50
- Six pounds heavier than Dave: d + 6
- A group of men and women: m + w
- The cost is marked up: c + m
What are some common word problem phrases for subtraction?
- Minus, difference, less than: -
- x minus five: x – 5
- The difference between Quentin’s and Rachel’s heights (if Quentin is taller): q – r
- Four pounds less than expected: e – 4
- The profit is the revenue minus the cost: P = R – C
What are some common word problem phrases for multiplication?
- The product of h and k: h * k
- The number of reds times the number of blues: r * b
- One fifth of y: (1/5) * y
- n persons have x beads each: total number of beads = nx
- Go z miles per hour for t hours: distance = zt miles
What are some common word problem phrases for ratios and division?
- Quotient, per, ratio, proportion: /
- Five dollars every two weeks (5 dollars/2 weeks) -> $2.5 per week
- The ratio of x to y: x/y
- The proportion of girls to boys: g/b
What are some common word problem phrases for average or mean?
- Sum of terms divided by the total number of terms
- Average of a and b: (a + b)/2
- Average salary of three doctors: (x + y + z)/3
What is the profitability formula?
Profits = P*Q – VC – FC
What is a conversion factor?
A fraction whose numerator and denominator have different units but the same value
How do you handle units in word problems?
(1) Add or subtract quantities with units: ensure that the units are the same, converting first if necessary
(2) Multiply quantities with units: multiply the units, cancelling as appropriate
(3) Convert from one unit to another: multiply by a conversion factor and cancel
What is the formula for rate or speed?
Rate = Distance / Time Speed = Distance / Time
Rate is measured in units of distance per unit of time (for example, miles per hour).
DERT: Distance Equals Rate Time. Notice, in addition, that the letters are in alphabetical order.
What is the formula for distance?
Distance = Rate * Time
What is the formula for Time?
Time = Distance / Rate
What is the formula for work completed?
Work Completed = Rate * Time
Rate is measured in units of output per unit of time (for example, 5 widgets produced per minute).
*HIDDEN CONSTRAINT: All work rates must be positive
How do you solve problems when people work together?
Add the rates – when two or more workers are performing the same task
Subtract the rates – when one worker is undoing the work of the other
What is the formula for total personal earnings?
Total Personal Earnings = Wage Rate ($/hr) * Hours Worked (hrs)
What is the formula for miles?
Miles = Miles per Hour * Hours Miles = Gallons per Hour * Gallons
What are integer constraints?
- Some word problems, by their nature, restrict the possible values of the variables
- The most common restriction is that variables must be integers (e.g. cars, marbles, people, etc. must be integers)
- BEWARE: hidden constraints show up in data sufficiency because information that seems like it should be insufficient on its own actually does provide an answer
How do you solve relative rate problems?
Create a third RT = D chart for the rate at which the distance between the bodies changes
(1) the bodies move towards each other
- e.g. two people decrease the distance between themselves at a rate of 5 + 6 = 11 mph
(2) the bodies move away from each other
- e.g. two cars increase the distance between the between themselves at a rate of 30 + 45 = 75 mph
(3) the bodies move in the same direction on the same path starting at the same point or different points
- e.g. person X and Y decrease the distance between themselves at a rate of 8 – 5 = 3 mph
How do you determine the average rate?
- If an object moves the same distance twice, but at different rates, then the average rate will NEVER be the average of the two rates given for the two legs of the journey
- In fact, because the object spends more time traveling at the slower rate, the average rate will be closer to the slower of the two rates than to the faster
- In order to find the average rate, you must first find the total combined time for the trips and the total combined distance for the trips (NOTE: you can actually pick any smart number for the distance)
How do you solve population growth or decay problems?
- Solve with a population chart – when some population increases by a common factor every time period
- Label the middle row “NOW” and work forward/backward, obeying any given conditions about the rate of growth or decay
- In some cases you might have to pick Smart Numbers for the starting point in your population
What is an Average (arithmetic mean)?
Average = S / n, where S is the sum of all of the terms in the set, n is the number of terms in the set, and A is the average.
What is a Weighted average?
In a weighted average, some data points contribute more than others to the overall average. This is in contrast to a regular average, in which each data point contributes equally to the overall average. A weighted average can be expressed with the formula A = [(D1)(W1) + (D2)(W2) + … + (Dn)(Wn)] / sum of weights, where each D represents a distinct data point, each W represents the weighting assigned to that data point, and A is the weighted average.