Question 969598
5 liters ANSWER

Let x=liters of 10% solution.

10 liters * 0.04 (4%)acid= 0.4 acid-liters  (it's a strange unit, but stay with me)

x liters * 0.10 (10%) acid= 0.1x acid-liters

I want a 6% solution (.06 acid) amount  is 10+x liters (10 from 4%, x from 10%)

(10+x) liters *.06%=0.06(10-x) acid-liters

Let's distribute that to get 0.6-0.06 x

Now add the first two solutions and let them equal the third

0.4 a-l +0.1x a-l=0.6 a-l-0.06x a-l

subtract 0.4 a-l from each side

0.1 x a-l=0.2 a-l +0.06x a-l
Subtract 0.06 a-l from each side

0.04x a-l=0.2 a-l
divide by 0.04  the units are acid only, so we will be left with liters as the units, which is what we want.

x=5 liters.

Let's see if this works
10 *(0.04) + 5 (.10)=.4 +.5=0.9    I am multiplying liters times per cent acid for the two.
15*(0.06)=0.9                           This is the final solution.

That checks