Math Tricks

Question 1

How do you predict if the sum, difference, or product will be ODD or EVEN?

Answer 1

Take simple even and odd numbers and plug them in

Question 2

How to recognize MULTIPLES of 2?

Answer 2

Last number is even

Question 3

How to recognize MULTIPLES of 3?

Answer 3

Sum of digits is multiple of 3

Question 4

How to recognize MULTIPLES of 4?

Answer 4

Last two digits are multiple of 4

Question 5

How to recognize MULTIPLES of 5?

Answer 5

Last digit is 5 or 0

Question 6

How to recognize MULTIPLES of 6

Answer 6

Sum of digits are multiple of 3, and last digits is even

Question 7

How to recognize MULTIPLES of 9

Answer 7

Sum of digits is multiple of 9

Question 8

How to recognize MULTIPLES of 10?

Answer 8

Last digit is 0

Question 9

How to recognize MULTIPLES of 12?

Answer 9

Sum of digits is a multiple of 3, last two digits are multiple of 4

Question 10

How to find a COMMON FACTOR of two numbers?

Answer 10

Break both numbers down to prime factors. Take only shared prime factors and multiply every combination possible. Each product is a factor. E.g. 3x3x5 = 3, 5, 3x3 or 9, 3x5 or 15, 3x3x5 or 45

Question 11

How to find LOWEST COMMON MULTIPLE of two numbers?

Answer 11

Prime factor both, multiply each factor by greatest number of times it appears in a prime factorization. E.g. 28 = (2x2)x7, and 42=2x3x7, is (2)2x(1)3x(1)7=84

Question 12

How to find standard deviation?

Answer 12

1. Find the mean of the set.
2. Find absolute value between each number and the mean.
3. Square each value
4. Average all squared values
5. Find square root of product

Question 13

What is 0!

Answer 13

1

Question 14

How to solve GROUP problem involving EITHER/OR CATEGORIES?

At a professional conference with 130, 94 are doctors, rest are dentists. If 48 are women, and 1/4 of dentists are women, how many are male doctors?

Answer 14

Make a grid with each column and row adding up to the total. Fill in cells with simple math and find the answer.

Question 15

How to solve a PERMUTATION problem with equal n and n slots?

Answer 15

n!

(e.g. if n=3, then 3x2x1=6)

Question 16

How to solve a PERMUTATION problem with n objects and k slots and order of arrangement matters?

Answer 16

n!

______

(n-k)!

Question 17

How to solve a COMBINATION problem with n objects and k slots where order of arrangement DOESN’T matter?

Answer 17

n!

______

k!(n-k)!

Question 18

How to solve a GROUP problem involving BOTH/NEITHER?

Of the 120 students at a school, 65 are studying French, 51 are studying Spanish, 53 are studying neither language. How many are studying both French and Spanish?

Answer 18

Group 1 + Group2 + Neither - Both = Total

65 + 51 + 53 - Both = 120

169 - Both = 120

Both = 49

Question 19

How to solve a CONDITIONAL PROBABILITY problem?

If 2 students are chosen at random to run an errand from a class of 5 girls and 5 boys, what is the probability that both students will be girls?

Answer 19

Multiply probabilities with each other.

5/10 x 4/9 = 20/90 or 2/9