Question 1125686
Let 4% solution be L (gal)   (For 'light' color)
Let 26% solution be D (gal)   (For 'dark' color)

.04 L + .26 D = .45  (because 15% of 3 gal is .45 gal of 'color'.

and 

L + D = 3

Multiply 1st eq by 100

4 L +26 D = 45

Mult 2nd eq by 4

4L + 4D = 12

subtract, and  22D = 33   D=1.5  and L = 1.5

Note, that since 15 = (4+26) /2  , I knew that the mix would be equal parts of each. The solution provides the full steps for any mixure problem worded this way.