The ratio is 1:7. This means there are 8 total parts. Start by dividing 40 by 8. This equals 5. Each part equals five, so the white marbles are 5. However, question asks for the number of RED. 7 of the 8 parts are red. 7*5 is 35 so there are 35 red marbles.
You write down the decimal fraction like say .75 you put .75/1. Then you multiply both sides by 100 so .75/1 would turn into 75/100. Then all you have to do is simplify the fraction.
