Rachel bought a total of 150 big and small chocolate bars. Then she decided to exchange 25% of her big chocolate bars for the small ones in the ratio of 1 : 4. After the exchange, she had 168 chocolate bars. What percentage of the chocolate bars she bought were small ones?

	Big chocolate bars	Small chocolate bars	Total
Before	4 u	?	150
Change	- 1 u
Comparing big and small chocolate bars at exchange	- 1 u	+ 4 u	+ 18
After	3 u	?	168

25% = ²⁵₁₀₀ = ¹₄

The number of big chocolate bars that were exchanged for small chocolate bars was repeated.  

Number of extra chocolate bars that Rachel received due to the exchange
= 4 u - 1 u
= 3 u

3 u = 168 - 150
3 u = 18

1 u = 18 ÷ 3 = 6

Number of small chocolate bars at first
= 150 - 4 u
= 150 - 4 x 6
= 150 - 24
= 126

Percentage of the chocolate bars Rachel bought that were small ones
=  ¹²⁶₁₅₀ x 100%
= 84% 

Answer(s): 84%