Maths problem: How to do, thanks?

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回答問題：
(a)
Let the original number of male and female members be 7k and 5k respectively where k is a positive constant.
Then, (7k - 6) : (5k - 3) = 5 : 4
(7k - 6)/(5k - 3) = 5/4
4(7k - 6) = 5(5k - 3)
28k - 24 = 25k - 15
3k = 9
k = 3
Number of male members = 7k = 21
Number of female members = 5k = 15

(b)
Total number of members
= (21 - 6) + (15 - 3)
= 15 + 12
= 27