Sarah and Simon collect pens and erasers. Simon has 60 less pens than erasers. Simon has 3 times the number of pens Sarah has, and Sarah has 25% more erasers than Simon. After Sarah gives Simon all of her pens and Simon gives Sarah 75% of his erasers, Simon has 24 more pens than erasers. Find the number of erasers Sarah has at first.

Sarah		Simon
Erasers	Pens	Erasers	Pens
3.75 u + 75	1 u	3 u + 60	3 u
+ 2.25 u + 45	- 1 u	- 2.25 u - 45	+ 1 u
6 u + 120	0	0.75 u + 15	4 u

100%+ 25% = 125% 

125% x 3 u
= ¹²⁵₁₀₀ x 3 u
= 3.75 u

125% x 60
= ¹²⁵₁₀₀ x 60
= 75 

75% x 3 u
= ⁷⁵₁₀₀ x 3 u
= 2.25 u  

75% x 60 
= ⁷⁵₁₀₀ x 60
= 45 

If another 24 erasers are given to Simon, he will have equal number of erasers and pens in the end.
4 u = 0.75 u + 15 + 24
4 u - 0.75 u = 39
3.25 u = 39

1 u = 39 ÷ 3.25 = 12

Number of erasers that Sarah has at first
= 3.75 u + 75
= 3.75 x 12 + 75
= 45 + 75
= 120
 
Answer(s): 120