Start with 10%
A percent is a number out of 100, so 10% is 10 out of 100. That is the same as 1 out of 10: one tenth.
So to find 10% of an amount, split the amount into 10 equal parts and take one of them. That is dividing by 10. To find 10% of 120, work out 120 ÷ 10 = 12.
The bar is the whole 120, and the scale under it runs from 0% to 100%. One tenth of the way along, the bar reads 12 and the scale reads 10%.
Build the rest from 10%
30% is 3 lots of 10%. 10% of 120 is 12, so 30% of 120 is 3 × 12 = 36.
Any multiple of 10% works the same way: find 10% first, then multiply by the number of tens. 20% of 120 is 2 × 12 = 24, and 70% of 120 is 7 × 12 = 84. 50% of 120 is 5 × 12 = 60, which is half of 120, because 50% is one half.
Three of the ten parts reach 36 on the bar and 30% on the scale: 30% of 120 is 36.
Half of 10%
5% is half of 10%. 10% of 120 is 12, so 5% of 120 is 6. Parts of the same amount can be added, so 25% of 120 is 10% + 10% + 5% = 12 + 12 + 6 = 30.
Check it another way: 25% is a quarter, and a quarter of 120 is 120 ÷ 4 = 30.
a percent is a hundredth, so n% of a quantity is that quantity × n/100
Make a quarter
The whole bar is 240, and it opens at 10%, which is 240 ÷ 10 = 24. Drag until a quarter of the bar is colored. That is 25%, and 10% + 10% + 5% = 24 + 24 + 12 = 60.
Three slips to avoid
30% of 120 is 36, not 84. 84 is 70% of 120, the part that was not taken.
It is not 30 + 120 = 150 either. A percentage of an amount is a share of it, so it is never more than the amount while the percentage is less than 100%.
And 12 is only 10% of 120, the first step. 30% needs three of those parts: 3 × 12 = 36.