A bag contains 3 blue, 4 red and 3 yellow marbles. How many blue marbles must be added to the bag for it to contain 75% blue marbles?

18

Step-by-step explanation:

Note that 75% = 75/100 = 3/4, so 3/4 of the marbles must be blue. Let x be amount of blue marbles added. We can write an equation using this.

75% = 3/4 = (3 + x blue marbles)/(3 + 4 + 3 + x total marbles) = (3+x)/(10+x)

Multiplying by 4(10 + x) on both sides:

3(10 + x) = 4(3+x)

30 + 3x = 12 + 4x

18 = x

Therefore, we must add 18 blue marbles for 75% of the marbles to be blue.

I hope this helps! :)