9 fewer pupils in the lower primary than upper primary signed up for a mass run. 2/3 of those registered were boys. 3/7 of the lower primary were girls. 1/4 of the pupils in the upper primary were girls. How many boys took part in the run?
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Question
Answer
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lower primary —
boys : girls : total
4u : 3u : 7u
upper primary —
boys : girls : total
3p : 1p : 4p
4u + 3p ——- (2/3) x (7u + 4p)
12u + 9p ——- 14u + 8p
14u – 12u = 2u ——- 9p – 8p = 1p
7u + 9 ——- 4p = 4 x 2u = 8u
1u ——- 9
1p ——- 2 x 9 = 18
4u + 3p ——- 4 x 9 + 3 x 18 = 90
Ans : 90 boys.