Mental Math · Multiplication · M4
You can halve one number and double the other without changing the product. Do it until one number becomes round and easy — turning something like 16 × 25 into 4 × 100.
Watch the method
16 × 25 — halve the 16, double the 25: 8 × 50. Again: 4 × 100 = 400. Each swap keeps the answer but makes the numbers friendlier.
Halve one, double the other — the answer stays the same.
Halve one, double the other — the answer stays the same.
Now it is an easy multiply.
Your turn
Use the method, then check. Stuck? Reveal the steps.
Cutting one factor in half and doubling the other leaves the product unchanged — but it can turn an awkward pair into a round one. Aim a factor at 10, 100, or another easy number.
Why it works
16 groups of 25 is the same as 4 groups of 100 — just rearranged.
14 × 5
Halve & double
7 × 10.
Easy now
7 × 10 = 70.
16 × 25
Step 1
8 × 50.
Step 2
4 × 100.
Easy now
4 × 100 = 400.
24 × 25
Step 1
12 × 50.
Step 2
6 × 100.
Easy now
6 × 100 = 600.
Halving BOTH numbers (or doubling both).
Halve one AND double the other. Doing the same to both changes the product.
Halving an odd number and getting stuck.
Stop when a factor turns odd, or when the other is already round. You don't have to reach 100 exactly.
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).16 × 25 = ?
12 × 5 = ?
14 × 5 = ?
8 × 15 = ?
16 × 50 = ?
24 × 25 = ?
18 × 5 = ?
12 × 35 = ?
28 × 25 = ?
32 × 25 = ?
16 × 35 = ?
24 × 15 = ?