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THINK:
- Number of girls did not change (adults left, boys joined).
- Total number of people increased by 50 = +125 boys - 75adults
- Percentages are just ratios in disguise

WORKING
               Girls:OldTotal  Girls:NewTotal
----------------------------------------------
Percentages       54%             36%
Ratio             54:100          36:100
Ratio            108:200         108:300 LCM(54,36)=2x54=3x36=108
Units           108u:200u       108u:300u

So NewTotal - OldTotal = 100u = 50.
Thus u = 0.5, Girls = 54, OldTotal = 100 and NewTotal = 150.

a) (NewTotal - OldTotal)/OldTotal = 50/100 = 50%
   Percentage increase in number of people is 50%.

b) There are 54 girls.


NOTES:
- By looking for the invariant property that did not change,
  in this case the number of girls, we can scale the ratios
  such that they become comparable.
- Students should learn to be proficient at computing LCMs
  (I think this is taught in school).
- Make a table to track things. It makes life easier. :-)
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