How many milliliters of a(n) 60% acid solution should we add to 300 milliliters of a(n) 90% acid solution to get a(n) 64.5% solution?

Solution:

X of a(n) 60% acid solution should be added

So

300*90%+x*60%=64.5%(x+300)

x=52.94

So 52.94ml  of a(n) 60% acid solution should be added