IQ Test Labs

Subtraction

Rounding-off:

• helps to smooth out difficult carry-overs.
• may be upwards or downwards (i.e not only subtraction).
• involves more steps than a single step, however they are all relatively easier.

Round-off one number and adjust other number(s).

Example: 1105 - 176

In this example round-off the first number. First subtract 105 from 1,105 to get 1,000. Then subtract 105 from 176 to get 71. Finally subtract 71 from 1,000 to get 929.

Worked answer:

1,105-105=1,000

176-105=71

1,000-71=929

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Round-off one number and adjust in the final step.

Example: 98 - 57

In this example round-off the first number, 98. Add 2 to get to 100, and then subtract 57 to get 43. Adjust by subtracting the initial 2, to make 41.

Worked answer:

100-57=43

43-2=41

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Round-off two or more numbers and adjust in the final step.

Example: 151 - 97

Subtract 1 from 151 to get 150 and add 3 to 97 to get 100. Add back 1 to the result. Add back 3, because 100 is 3 less than 97.

Worked answer:

150-100=50

50+1+3=54

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Subtraction by five

For numbers where the units digits are smaller than five, it may be easier to first add five, and then subtract ten.

Example: 23

Instead of 23-5=18, add five then subtract ten.

Worked answer:

23+5=28

28-10=18

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Subtracting from one thousand

Subtract the hundreds and tens digits by nine; then subtract the units digit by ten.

Example: 451

Subtract, in order, from the hundreds digit to the units digit i.e from left to right.

Worked answer:

9-4=5

9-5=4

10-1=9

549