There are 130 red, blue and green marbles in a bag. the ratio of red to blue to green is 5: 3: 2 respectively. how much marbles in the bag are blue?

One would take the ratios, and add them. 
5+3+2 = 10
The divide the total number of marbles by the sum of the ratios.
130/10 = 13
And finally multiply the quotient by each of the three numbers in the ratio (5:4:3) to get the number of marbles that are each colour. 
Red: 13 * 5 = 65
Blue: 13 * 4 = 52
Green:  13 * 3 = 39 
Therefore, there are 52 blue marbles in the bag.