Apainter is making a new color she used color a, color b and color c. in the ratio 2: 5: 6. she used 91 gallons of paint. how much of color she use?

You can solve by putting this into an equation:

2x + 5x + 6x = 91

The x represents the amount of color times its ratio

13x = 91
x = 7

So now we have:
2(7) + 5(7) + 6(7) = 91
14 + 35 + 42 = 91
49 + 42 = 91

91 = 91