Basic, lesson 1 of 3
Mental math that holds up under pressure
A few tricks cover most case arithmetic: flip percents, divide by multiplying, and count zeros separately from digits.
8 min, 3 checks
1Percents and fractions
- Flip a percent: 18% of 50 is the same as 50% of 18, which is 9.
- Know the fractions: 12.5% is 1/8, 16.7% is 1/6, 33.3% is 1/3, 37.5% is 3/8, 62.5% is 5/8.
- Build from 10% and 1%: 23% of 400 is 40 + 40 + 12 = 92.
Check 1
What is 16% of 25?
Check 2
What is 37.5% of 640?
2Zeros and near numbers
Split a number into digits and powers of ten. $4.8 billion divided by 120 million is 4.8 / 1.2 = 4, times 10 to the 9 minus 8 = 10, so 40. Say the zeros out loud: "billions over millions is a thousand, so 4.8 thousand over 120."
For awkward multiplications, round and correct: 49 x 52 = 50 x 52 - 52 = 2,600 - 52 = 2,548.
Check 3
What is $3.6 million divided by 0.04?
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