Answer:
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! :)
