Mental Math · Level Up · L3
Numbers near 100 lean on 100, exactly like 9 and 11 lean on 10. ×99 is ×100 minus one copy; ×101 is ×100 plus one copy. Multiplying by 100 is trivial, so the whole thing is one easy adjustment.
Watch the method
47 × 99 = 47 × 100 − 47 = 4700 − 47 = 4653. And 47 × 101 = 4700 + 47 = 4747. Multiply by 100, then add or subtract one copy.
Multiply by 100 first — just add two zeros.
99 is one less than 100, so subtract one 47.
Your turn
Use the method, then check. Stuck? Reveal the steps.
×100 just appends two zeros. 99 is one short of 100 (subtract one copy); 101 is one over (add one copy); 98 is two short; 102 is two over. Same idea as ×9 and ×11, one level up.
Why it works
Buying 99 of something costs 100 of them minus one.
23 × 99
×100
23 × 100 = 2300.
Subtract one copy
2300 − 23 = 2277.
47 × 101
×100
47 × 100 = 4700.
Add one copy
4700 + 47 = 4747.
47 × 98
×100
47 × 100 = 4700.
Subtract two copies
4700 − 94 = 4606.
Subtracting 100 instead of one copy of the number.
47 × 99 = 4700 − 47, not 4700 − 100.
Adjusting the wrong way for ×101.
101 is MORE than 100, so you ADD a copy: 4700 + 47.
12 problems across three tiers. Auto-graded on submit. Hints and a full walkthrough on every question. Your work is saved on this device — sign in to keep it across devices.
t₁ = 5, d = 3 and t1=5,d=3 both count. For a list of numbers, separate them with commas (e.g. 3, 10, 17).47 × 99 = ?
23 × 99 = ?
56 × 99 = ?
38 × 101 = ?
64 × 101 = ?
47 × 101 = ?
72 × 99 = ?
85 × 101 = ?
99 × 99 = ?
47 × 98 = ?
63 × 102 = ?
28 × 99 = ?