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