Decimals & Percents · Percent of a number · 6.2

Mental percents.

Find 10% by moving the point one place left; 1% by moving two. Then build other percents: 5% is half of 10%, 20% is double, 15% is 10% + 5%. 10% of 70 = 7.

01 / THE IDEA

See it work.

Watch the method

Mental percents

10% of 70 = 7 (shift the point one place).

1 / 2
10% of 7010\%\text{ of }70
1

Turn the percent into a decimal

10% = 0.1.

10%=0.110\% = 0.1
2

Multiply by the number

“Of” means multiply.

0.1×70=70.1 \times 70 = 7
02 / YOUR TURN

Practice until it feels automatic.

Your turn

Work it out.

Type a decimal, e.g. 0.75.

streak 0
best 0
… =
— / WHY IT WORKS

The idea underneath.

Build from 10% and 1%

10% = ÷10, 1% = ÷100. Combine and scale them: 30% = 3 × 10%, 5% = ½ × 10%.

Why it works

10% of 70 is 7; 5% is 3.5.

03 / WORKED EXAMPLES

From easy to tricky.

EX 1easy

What is 20% of 50?

1

Turn the percent into a decimal

20% = 0.2. 20% = 0.2

2

Multiply by the number

“Of” means multiply. 0.2 × 50 = 10

EX 2medium

What is 20% of 60?

1

Turn the percent into a decimal

20% = 0.2. 20% = 0.2

2

Multiply by the number

“Of” means multiply. 0.2 × 60 = 12

EX 3hard

What is 20% of 40?

1

Turn the percent into a decimal

20% = 0.2. 20% = 0.2

2

Multiply by the number

“Of” means multiply. 0.2 × 40 = 8

04 / COMMON TRAPS

Where students slip.

Trap

Shifting the point the wrong way.

Fix

10% means ÷10 — point moves left.

Trap

Not reusing 10% to build others.

Fix

20% = double 10%, 5% = half.

05 / QUIZ

Test yourself.

The Test

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.

0/12Answered
How to answerType just the value. Formatting won't trip you up — we ignore spaces, capital letters, commas, and subscript style, so 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).
Core

Core (4)

C1

What is 1% of 190?

Need a hint?
  • 10% = move the point one place left.
  • Scale it for other percents.
C2

What is 1% of 190?

Need a hint?
  • 10% = move the point one place left.
  • Scale it for other percents.
C3

What is 5% of 150?

Need a hint?
  • 10% = move the point one place left.
  • Scale it for other percents.
C4

What is 10% of 150?

Need a hint?
  • 10% = move the point one place left.
  • Scale it for other percents.
Apply

Apply (4)

A1

What is 1% of 180?

Need a hint?
  • 10% = move the point one place left.
  • Scale it for other percents.
A2

What is 5% of 90?

Need a hint?
  • 10% = move the point one place left.
  • Scale it for other percents.
A3

What is 10% of 20?

Need a hint?
  • 10% = move the point one place left.
  • Scale it for other percents.
A4

What is 10% of 80?

Need a hint?
  • 10% = move the point one place left.
  • Scale it for other percents.
Extension

Extension (4)

E1

What is 10% of 130?

Need a hint?
  • 10% = move the point one place left.
  • Scale it for other percents.
E2

What is 5% of 80?

Need a hint?
  • 10% = move the point one place left.
  • Scale it for other percents.
E3

What is 5% of 130?

Need a hint?
  • 10% = move the point one place left.
  • Scale it for other percents.
E4

What is 5% of 150?

Need a hint?
  • 10% = move the point one place left.
  • Scale it for other percents.