Speed Mathematics Flashcards
What is the easiest way to substract 9?
Take away 10, then add 1 to the answer.
What is the easiest way to substract 8?
Take away 10, then add 2 to the answer.
What is the easiest way to substract 90?
Take away 100, then add 10 to the answer.
What is the easiest way to substract 80?
Take away 100, then add 20 to the answer.
What is the easiest way to substract 70?
Take away 100, then add 30 to the answer.
What is the easiest way to substract 7?
Take away 10, then add 3 to the answer.
78 / 2
80 - 2. Half of (80 - 2) is (40 - 1). That is your answer.
40 - 1 = 39
38 x 2
Think of (40 - 2). Double would be (80 - 4) = 76.
To add 9 …
…add 10 and substract 1.
To add 8 …
…add 10 and substract 2.
To add 7 …
…add 10 and substract 3.
To add 47 …
…add 50 and substract 3.
To add 196 …
…add 200 and substract 4.
To add 38 …
…add 40 and substract 2.
To add 288 …
…add 300 and substract 12.
Addition: If the units digit is high, …
…round of to the next ten and then substract the difference.
Addition: If the units digit is low, …
…add the tens, then the units.
$4.95 + $6.95 + $13.95
$5 + $7 + $14
- 15¢
= $25.85
495 + 695 + 1395
500 + 700 + 1400
-15
= 2585
8461 + 5678
8 + 3 = 13 (13,000)
4 + 6 = 10 (14,000)
+ 61 (14,061)
+ 78 (add 100 and substract 20, then substract 2)
= 14,139
6 + 8 + 4
6 + 4 = 10
+ 8
= 18
When adding a column of numbers, …
…add pairs of digits to make tens first, than add the other digits.
463 - 87
436 - 100 = 336
+ 13 (add 10 + 3)
= 349
134 - 76
24 + 34 = 58
A good general rule for dividing by factors is to …
…divide by the smaller number first and then by the larger number.
3,444 / 21
3,444 / 3 = 1,148
1,148 / 7 = 164
To divide by a two-digit number ending in 5, …
…double both numbers and use factors.
29 x 8
30 x 8 = 240
240 - 8 = 232
The general rule for standard long division
Round off the divisor to the nearest ten, hundred or thousand to make an easy estimate.
This way you’re able to make an easy estimate and then proceed in the usual way to correct as you go.
If you’re dividing by 31, …
…round of to 30 and divide by 3 and 10.
If you’re dividing by 87, …
…round of to 90 and divide by 9 and 10.
If you’re dividing by 321, …
…round of to 300 and divide by 3 and 100.
If you’re dividing by 487, …
…round of to 500 and divide by 5 and 100.
If you’re dividing by 6,142, …
…round of to 6,000 and divide by 6 and 1000.
To convert 80kg to pounds…
…calculate 80 x 2.2
double 80 to get 160,
then multiply this by 11 (1,760),
and divide by 10 to get 176 pounds
90kg in pounds
90 x 2 = 180
180 x 11 = 1,980
1,980 / 10 = 198 pounds
An easy way to divide any two-digit number by 9 is…
…to take the tens digit as the answer, and add both digits of the number for the remainder.
42 / 9
= 4 r6
34 / 9
= 3 r7
71 / 9
7 r8
Often, when you have to multiply two number, the calculation can be made easier if you…
…can double one number and halve the other.
3 x 14
= 6 x 7
= 42
An easy way to divide by 15 is to …
…double the number and divide by 30.
An easy way to divide by 25 is to …
…double the number and divide by 50.
An easy way to divide by 35 is to …
…double the number and divide by 70.
An easy way to divide by 45 is to …
…double the number and divide by 90.
2,341 / 35
= 2,341 x 2 = 4,682
= 4,682 / 10 = 468.2
= 468.2 / 7 = 66.8857
20000000 - 52316
20000000
-0052316
= 19947684
To change Fahrenheit to Celsius, …
…substract 30 degrees and then halve*
*close enough for practical purposes.
To change Celsius to Fahrenheit, …
…double and then add 30*.
*close enough for practical purposes.
One foot is …cm.
…approximately 30cm.
One inch is …cm.
…approximately 2.5cm.
1 mile is …km.
…approximately 1.6km.
1 km is …miles.
…approximately 0.6 miles.
1kg is …pounds.
…approximately 2.2 pounds.
1 pound is …kg
…approximately 0,45kg.
