PSLE
Four girls, Gwen, Gem, Natalie and Mary raised money for a charity. Gwen and Gem raised a total of $65. Together, Gem, Natalie and Mary raised a total of $100. The total amount of money raised by all 4 girls is 4 times the amount that Gem raised. How much money did Natalie and Mary raise?
Amount that Gem and Gwen raised = $65
Amount that Gem, Natalie and Mary raised = $100

Gem is the repeated identity.

Gem + Gwen = 65 --- (1)
Gem + Natalie + Mary = 100 --- (2)

Amount that Gem raised = 1 u
Amount that 4 girls raised = 4 u

(1) + (2)
(Gem + Gwen) + (Gem + Natalie + Mary) = 65 + 100
Gem + 4 girls = 165
1 u + 4 u = 165
5 u = 165

1 u = 165 ÷ 5 = 33

Total amount that Natalie and Mary raised
= 100 - 1 u
= 100 - 33
= $67

Answer(s): $67