Lucy made sandwiches for sale. 60% of the sandwiches were egg sandwiches and the rest were sardine and mushroom sandwiches in the ratio 4 : 1. If a customer bought 70% of the egg sandwiches and all of the mushroom sandwiches, what percentage of the sardine sandwiches must Lucy sell for her to maintain the original percentage of egg sandwiches at first? Leave your answer in decimals and correct to 1 decimal place.

Egg sandwiches	Sardine sandwiches	Mushroom sandwiches
3 u	2 u
	1.6 u	0.4 u

	Egg sandwiches	Sardine sandwiches	Mushroom sandwiches	Total sandwiches
Before	3 u	1.6 u	0.4 u	5 u
Change	- 2.1 u	- 1 u	- 0.4 u
After	0.9 u	0.6 u	0	1.5 u

60% = ⁶⁰₁₀₀ = ³₅
70% = ⁷⁰₁₀₀ = ⁷₁₀

Total number of sandwiches = 5 u

Number of sardine and mushroom sandwiches
= 5 u - 3 u
= 2 u

Number of sardine sandwiches at first
= ⁴₅ x 2 u
= 1.6 u

Number of mushroom sandwiches at first
= 2 u - 1.6 u
= 0.4 u

Number of egg sandwiches sold
= 70% x 3 u
= ⁷⁰₁₀₀ x 3 u
= ⁷₁₀ x 3 u
= 2.1 u

Number of mushroom sandwiches sold
= ¹₅ x 2 u
= 0.4 u

Number of egg sandwiches left 
= 3 u - 2.1 u
= 0.9 u

To maintain the percentage of the egg sandwiches in the end, 
60% → 0.9 u
100% → ^0.9₆₀ x 100 = 1.5 u

Total number of sandwiches in the end to maintain the original percentage of egg sandwiches = 1.5 u

Number of sardine sandwiches left
= 1.5 u - 0.9 u
= 0.6 u

Number of sardine sandwiches that Lucy must sell to maintain the original percentage of egg sandwiches
= 1.6 u - 0.6 u
= 1 u

Percentage of the sardine sandwiches that Lucy must sell
= ¹_1.6 x 100%
= 62.5%

Answer(s): 62.5%