Question 909462
We have 24 qt of 20% acid
We want to add some 100% acid to make   40% acid
1*x+0.2*24=0.4(24+x)
1*x+4.8=9.6+0.4x
1x-0.4x=9.6-4.8
0.6x=4.8
x=8 qt at 100% (8) is added to 24 qt at 20%(4.8) to make 40% of 32 qt(12.8)
check
1*8+0.2*24=0.4(24+8)
8+4.8=0.4(32)
12.8=12.8
ok