90% and 97% pure

Let the quantity of 90% acid solution be x litres.

∴ quantity of 97% acid solution = (21 - x) litres

Now,

90% of x + 97% of (21 - x) = 95% of 21

⇒ 7x = 21(97 - 95)

⇒ 7x = 42

⇒ x = 6

Thus, 90% acid solution = 6 litres

97% acid solution = 21 - 6 = 15 litres