let

x = liters of 35% acid solution

y = liters of 85% acid solution

we need to produce 80 liters of a 60% acid solution thus

x + y = 80

0.35x + 0.85y = 0.60*80

by solving the above system of equations we find

x = 40 l

y = 40 l

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40 liters of 35% acid solution and 40 liters of 85% acid solution must be used.