Maths shortcuts

Multiply big numbers without writing a single line of working.

• Multiply by 11Split the digits apart and drop their sum in the middle.
• Multiply by 5Halve the number, then add a zero.
• The 9 times table on your fingersFold one finger and read the answer off both hands.
• Multiply by 25Divide by 4 and add two zeros.
• Multiply two numbers near 100Work out how far each is below 100 and let those gaps do the work.
• Same tens digit, units adding to 10When 43 × 47 shows up, the answer takes five seconds.

Round, adjust, done — no borrowing.

• Subtract from 100 or 1000All from 9, and the last from 10 — no borrowing at all.
• Add by rounding, then adjustRound the awkward number up, add easily, then give back the difference.