Stage 6 · Big numbers & speed
Multiplying near 100
Front: cross-subtract. Back: gap × gap.
How to do it
Method
Front: cross-subtract. Back: gap × gap
96 × 97: 96 − 3 = 93, 4 × 3 = 12 → 9312
Another way
Column multiplication
96 × 7 = 672, 96 × 90 = 8640, 672 + 8640 = 9312
In full
96 × 97: each is 4 and 3 below 100. Front = 96 − 3 = 93 (or 97 − 4, same answer), back = 4 × 3 = 12 → 9312. The back is always two digits: 99 × 98 → 97 and 02 → 9702; if it runs past two digits, carry: 89 × 89 → 7800 + 121 = 7921. Multiplying in columns gets the same answer too; near 100 this is far quicker.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
91 × 91
Show the answer and working
Answer: 91 × 91 = 8281
91 × 91: 91 is 9 short of 100, 91 is 9 short. Front 91 − 9 = 82, back 9 × 9 = 81, front then back → 8281
Example 2
93 × 98
Show the answer and working
Answer: 93 × 98 = 9114
93 × 98: 93 is 7 short of 100, 98 is 2 short. Front 93 − 2 = 91, back 7 × 2 = 14, front then back → 9114
Example 3
89 × 99
Show the answer and working
Answer: 89 × 99 = 8811
89 × 99: 89 is 11 short of 100, 99 is 1 short. Front 89 − 1 = 88, back 11 × 1 = 11, front then back → 8811
Example 4
93 × 99
Show the answer and working
Answer: 93 × 99 = 9207
93 × 99: 93 is 7 short of 100, 99 is 1 short. Front 93 − 1 = 92, back 7 × 1 = 7, the back is always two digits: 7 is written 07, front then back → 9207
Practise multiplying near 100 in the game →Stage 6 · Big numbers & speed · Free to start, unlimited questions, no download