Skip to content

Alerts Center

New quiz added: Multiplication Tables — try it now

Fractions lesson updated with new practice worksheets

This month's resource pack is now live

View All Alerts

Messages

Tip: Create a free account to save your quiz progress

New here? Check out our Getting Started guide

New Sudoku puzzles added this week

View All Messages

Multiplication Tricks

Multiplying by 15 – Times 10 Plus Half

Fifteen is 10 + 5, so you get the answer by adding a “times 10” and a half of it.

This page explains the rule, shows why it works, and walks through worked examples. Then you can try it yourself and check your understanding with practice questions. Use the previous and next links at the bottom to move through the tricks in order.

The rule

The Rule: To multiply by 15, multiply the number by 10 and add half of that result.

Why it works

Why It Works: 15 is simply 10 plus half of 10. So once you have multiplied by 10 — an instant step — all that is left is to add half of that very same answer, which is usually easy to work out in your head.

Formal Proof (for advanced learners)

15 = 10 + 5 = 10 + (10 ÷ 2), so n×15 = n×10 + n×(10÷2) = 10n + (10n)÷2, by the distributive law.

Examples

  • 40 × 15 → 400 + 200 = 600.
  • 12 × 15 → 120 + 60 = 180.
  • 50 × 15 → 500 + 250 = 750.

Step by step

Example A: 18 × 15

  1. Multiply by 10: 18 × 10 = 180.
  2. Half of that: 180 ÷ 2 = 90.
  3. Add them: 180 + 90 = 270.

Example B: 46 × 15

  1. Multiply by 10: 46 × 10 = 460.
  2. Half of that: 460 ÷ 2 = 230.
  3. Add them: 460 + 230 = 690.

Try it yourself

Now it is your turn. A new random question appears each time. Type your answer in the box and press Check Answer. If you are right, we show how the trick got you there, step by step. If you are not, you will see the full working two ways, the shortcut and the usual method, so you can spot exactly where your answer differed. Press New Question as often as you like: a few quick rounds is the best way to make the trick feel automatic.

Try It Yourself

Practice questions

Have a go on paper first, then tap Show answer to check your method.

  1. Work out 12 × 15.

    Show answer

    180. Multiply by 10: 12 × 10 = 120. Half of that: 120 ÷ 2 = 60. Add them: 120 + 60 = 180.

  2. Work out 26 × 15.

    Show answer

    390. Multiply by 10: 26 × 10 = 260. Half of that: 260 ÷ 2 = 130. Add them: 260 + 130 = 390.

  3. Work out 44 × 15.

    Show answer

    660. Multiply by 10: 44 × 10 = 440. Half of that: 440 ÷ 2 = 220. Add them: 440 + 220 = 660.

  4. Work out 80 × 15.

    Show answer

    1200. Multiply by 10: 80 × 10 = 800. Half of that: 800 ÷ 2 = 400. Add them: 800 + 400 = 1200.

  5. A lesson lasts 15 minutes. How many minutes do 32 lessons take?

    Show answer

    480. Multiply by 10: 32 × 10 = 320. Half of that: 320 ÷ 2 = 160. Add them: 320 + 160 = 480.

Tips and watch-outs

  • It works because 15 = 10 + 5, and n × 5 is half of n × 10.
  • Odd numbers still work: 13 × 15 = 130 + 65 = 195.