Ratios and percent of a number
A ratio simplifies like a fraction; a percent of an amount is that percent written as a decimal, times the amount.
A worked example
Simplify 9:15 by dividing both parts by 3, giving 3:5. To find 40% of 70, work out 0.40 x 70 = 28.
The mistake people actually make
A ratio is not a fraction of the whole: 4 boys to 6 girls means 4 out of 10 children are boys, not 4 out of 6.