One way to solve it is by making 3 equations and solve them simultaneously. But there is rather easier way tosolve it using back tracking.

It’s given that at the end, each had 16 pens (16, 16, 16) i.e. after C gave pens to A & B as many as they had. It means that after getting pens from C their pens got doubled. So before C gave them pens, they had 8 pens each and C had 32 pens i.e. (8, 8, 32).

Similarly, before B gave pens to A & C, they had 4 & 16 pens respectively and B had 42 pens i.e. (4, 28, 16)

Again, before A gave pens to B & C, they had 14 & 8 pens respectively and A had 39 pens i.e. (26,14, 8)

Hence, initially A had 26 pens, B had 14 pens and C had 8 pens.