Here’s my way to explain the problem:
Let no. Boys be B and no. Girls be G
B + G = 350 (A)
1/5 G + 1/4 B = 80(B)
This is an alternative form of equation B called B*:
G + 5/4B = 400(B*)
If solve the equations A and B* simultaneously B* – A, we get:
1/4 B = 50
Comparing with equation A, 1/5 G = 30
since we have 1/5G = 30 and 1/4 B = 50
G = 150 and B = 200
150 – 30 = 120
120 girls did not attend.
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