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25/80 + 24/80 = 49/80. 16 and 20 both go into 80. !6 goes in 5 times so you multiply 5 by 5. 20 goes into 80 4 times so you multiply 4 by 4. 24/80 + 25/80 = 49/80.
84 blue marbles.
Steps:
1) Use the 3:7 ratio
2) Since there are 36 red marbles, you should know to multiply the ratio by 12.
3) Multiply 7 by 12 (7x12=84)