Xuan made buns for sale. 40% of the buns were mushroom buns and the rest were beef and chicken buns in the ratio 4 : 1. If a customer bought 70% of the mushroom buns and all of the chicken buns, what percentage of the beef buns must Xuan sell for her to maintain the original percentage of mushroom buns at first? Leave your answer in decimals and correct to 1 decimal place.

Mushroom buns	Beef buns	Chicken buns
2 u	3 u
	2.4 u	0.6 u

	Mushroom buns	Beef buns	Chicken buns	Total buns
Before	2 u	2.4 u	0.6 u	5 u
Change	- 1.4 u	- 1.5 u	- 0.6 u
After	0.6 u	0.9 u	0	1.5 u

40% = ⁴⁰₁₀₀ = ²₅
70% = ⁷⁰₁₀₀ = ⁷₁₀

Total number of buns = 5 u

Number of beef and chicken buns
= 5 u - 2 u
= 3 u

Number of beef buns at first
= ⁴₅ x 3 u
= 2.4 u

Number of chicken buns at first
= 3 u - 2.4 u
= 0.6 u

Number of mushroom buns sold
= 70% x 2 u
= ⁷⁰₁₀₀ x 2 u
= ⁷₁₀ x 2 u
= 1.4 u

Number of chicken buns sold
= ¹₅ x 3 u
= 0.6 u

Number of mushroom buns left 
= 2 u - 1.4 u
= 0.6 u

To maintain the percentage of the mushroom buns in the end, 
40% → 0.6 u
100% → ^0.6₄₀ x 100 = 1.5 u

Total number of buns in the end to maintain the original percentage of mushroom buns = 1.5 u

Number of beef buns left
= 1.5 u - 0.6 u
= 0.9 u

Number of beef buns that Xuan must sell to maintain the original percentage of mushroom buns
= 2.4 u - 0.9 u
= 1.5 u

Percentage of the beef buns that Xuan must sell
= ^1.5_2.4 x 100%
= 62.5%

Answer(s): 62.5%