The easiest way to understand this question is to give both fractions the same denominator. Only one fraction? Nope, it says the pie is split into 15 pieces, making 15 and 3 the denominators. As for the numerators, the question has given you only one, and is asking for the numerators above 15 (aka, the number of pieces you need colour).
With this in mind, all you need to do is set up a ratio:
2/3 = ?/15
Well how do you get 3 x ? = 15? Rearrange the equation to 15/3 = 5
So now we know to times the demonimator by 5 to get 15, and to keep the ratio the same we must do the exact same thing to the numerator.
2 x 5 = 10
That leaves us with 2/3 = 10/15
Therefore, you should colour 10 pieces.
