Math problems mixture

Question

How many ml of 10 percent acid should be added to pure acid to make 40 ml of 25 percent acid?

Answer 1

10X + 100(40-X) = 25(40)

10X + 4000-100X = 1000

90X = 3000

X =300/9 = 33.3 ml of 10 percent mixed with 66.7 ml of pure acid give a mixture of 40ml of 25% acid

rounded off to one decimal place

exact amount is 33 1/3 ml of 10% and 66 2/3 ml of pure acid