In an elementary school, 70% of the 1st graders are girls and 30% of the 2nd graders are girls. if 40% of all first and second graders are girls, then what is the ratio of 1st graders to 2nd graders?
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Let F= no. of 1st graders and S=no. of 2nd graders.

0.7F+0.3S=0.4(F+S) (total girls).

Divide through by S: 0.7F/S+0.3=0.4(F/S+1)

The ratio of first to second graders is r=F/S so 0.7r+0.3=0.4r+0.4.

0.7r-0.4r=0.4-0.3=0.1, 0.3r=0.1, r=0.1/0.3=1/3 so the ratio is 1:3.

by Top Rated User (982k points)

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