if n1 kg of a% alloy is mixed with n2 kg of b% alloy
then resultant alloy will have C%=(a*n1+b*n2)/(n1+n2)% alloy of n1+n2 kg.
let n kg of 60% alloy is taken
and 1 kg of 100%(pure copper) is taken
if mixed, we get C= 70% alloy.
i think you can do it now