There are 320 pupils there are 24 more girls than boys how many boys are there

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Let b = boys and g = girls. We have two equations:

(1) b + g = 320
(2) g = b + 24

Substitute (2) into (1) for g
b + (b + 24) = 320

Combine b terms:
2b + 24 = 320

Use our equation calculator:
b = 148

Substitute b = 148 into (2)
g = 148 + 24
g = 172