How do I do this question

Question

A container has 2litresof 10% strength cordial. How much pure cordial needs to be added to make the strength 25%?

Answer 1

2 (.1) + x(1) = (2+x)(.25)

.2 + x = .5 + x/4

3x/4 = .3

x/4 = .1

x = .3(4/3) = .4

x = .4 liters of pure strength 100% needed to make .1 to .25, 10% 2 liters to 25% 2.4 liters

10% is within 15 from 25, 100 is within 75 of 25

75 is 5 times 15

2 is 5 times .4 you need 5 times as much 10% as 100% to get 25%