Let
x -----> number of liters of 10% acid solution
y ----> number of liters of 70% acid solution
so
we have that
x+y=90 ------> x=90-y -----> equation 1
Remember that
10%=10/100=0.10
70%=70/100=0.70
20%=20/100=0.20
so
0.10x+0.70y=0.20(90) ------> equation 2
Solve the system of equations
substitute equation 1 in equation 2
0.10(90-y)+0.70y=0.20(90)
solve for y
9-0.10y+0.70y=18
0.60y=18-9
0.60y=9
y=15
Find out the value of x
x=90-y
x=90-15=75
therefore
the answer is
number of liters of 10% acid solution is 75 and number of liters of 70% acid solution is 15