Answered

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?

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Cleopatrabear122id
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. 
AL2006id
5 parts are red
3 parts are blue
2 parts are green

There are 10 parts in the bag altogether, and they add up to 130 marbles.
So each 'part' is 13 marbles.

Red = 5 parts = 65 marbles
Blue = 3 parts = 39 marbles
Green = 2 parts = 26 marbles

65 + 39 + 26 = 130 so we ended up on the track.