 How to Do Mental Math for the ASVAB - dummies

Make your life easier and practice mental math for the ASVAB. Have you ever envied those people who can perform calculations on large numbers in their heads? You can be one of those people. That’s right. All you have to do is practice the distributive property of math.

The distributive property, often referred to as the distributive law of math, lets you separate or break larger numbers into parts for simpler arithmetic. It basically says that a(b + c) is the same as (a × b) + (a × c).

Suppose you want to mentally multiply 4 by 53.

4 × 53 is the same as (4 × 50) + (4 × 3). Four times 50 is easy; it’s 200. Four times 3 is also easy. It’s 12. Two hundred plus 12 is 212.

Try another one with a bit of a twist.

Mentally perform the calculation 12 × 19.

12 × 19 is equivalent to 12(20 – 1) = (12 × 20) – (12 × 1).

You can quickly mentally calculate that 12 times 20 is 240, and that 12 times 1 is 12. Subtract 12 from 240, and you have 228.

If 12 × 20 is still too large for mental calculation, you can break it down to (12 × 10) + (12 × 10), or 120 + 120.

You can also use the distributive property for division, although that takes a bit more practice: 340 ÷ 4 is the same as (340 ÷2) ÷ 2. You can quickly calculate that 340 divided by 2 is 170, and 170 divided by 2 is 85.

And you can express 340 ÷ 4 as (100 ÷ ds 4) + (100 ÷ 4) + (100 ÷ 4) + (40 ÷ 4). You can mentally calculate 100 divided by 4 as 25. Forty divided by 4 is also easy — it’s 10. So 25 + 25 + 25 + 10 = 85. Keep practicing, and you’ll be known as the neighborhood lightning calculator.

You’re not allowed to use calculators on the math subtests of the ASVAB, so practicing the distributive property can be a big timesaver. 