Diana bought a total of 128 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 : 3. After the exchange, she had 144 chocolate bars. What percentage of the chocolate bars she bought were small ones?

	Big chocolate bars	Small chocolate bars	Total
Before	4 u	?	128
Change	- 1 u
Comparing big and small chocolate bars at exchange	- 1 u	+ 3 u	+ 16
After	3 u	?	144

25% = ²⁵₁₀₀ = ¹₄

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

Number of extra chocolate bars that Diana received due to the exchange
= 3 u - 1 u
= 2 u

2 u = 144 - 128
2 u = 16

1 u = 16 ÷ 2 = 8

Number of small chocolate bars at first
= 128 - 4 u
= 128 - 4 x 8
= 128 - 32
= 96

Percentage of the chocolate bars Diana bought that were small ones
=  ⁹⁶₁₂₈ x 100%
= 75% 

Answer(s): 75%