1 grp of g is 5 more g than 1 grp of b

one grp
b: 1u
g: 1u + 5


nos.
b: 1u x 4 = 4u
g: (1u + 5) x 5 = 5u + 25
total = 9u + 25

5 x 4u = 3 x (5u + 25)
20u = 15u + 75
1u = 15


total = 160

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Thanks @Snell. Can you explain the last 3 lines, ie:

5 x 4u = 3 x (5u + 25) —— I don’t understand this section’s working.

Thank you.

nos.
b: 1u x 4 = 4u
g: (1u + 5) x 5 = 5u + 25

&
the qn states "3/8 of all pupils ... were boys"

so b : g = 3 : 5 , right?

hence ,
5 x 4u = 3 x (5u + 25)
1u = 15

OR
if u prefer:
"3/8 of all pupils ... were boys"
means b is 3/8 of total


nos.
b: 1u x 4 = 4u
total = 9u + 25

4u = 3/8 x (9u + 25)
4u = 27u/8 + 75/8
5u/8 = 75/8
5u = 75
1u = 15

same ans.

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I understand the second style of working better. Thank you again for your clarifications and help!

it's ok.

in fact, the 2 methods r the same

:)