Tell if a number divides by 4
Ignore everything but the last two digits. If they divide by 4, the whole number does, however big.
- Look only at the last two digits: 7,316 → 16.
- 16 ÷ 4 = 4, so 7,316 divides by 4.
- For 2,354, the last two digits are 54, which doesn't divide by 4, so neither does 2,354.
Tip: Shortcut for the last two digits: if the tens digit is even, the last digit must be 0, 4 or 8; if it's odd, 2 or 6.
Try it now: Which of these divide by 4: 1,932, 5,618, 3,000? [1,932 and 3,000 do; 5,618 doesn't.]
Next crumbs
- 🧮 Mental mathTell if a number divides by 3Add up the digits. If that sum divides by 3, so does the whole number, however long it is.
- 🧮 Mental mathMultiply by 25: divide by 4, then add two zerosFour quarters make a dollar. So 36 × 25 is 36 ÷ 4 = 9, then two zeros: 900.
- 🧮 Mental mathConvert Celsius to Fahrenheit in your headDouble it, knock off a tenth, add 32. That's the exact formula, just in friendly pieces.