Write a balanced overall reaction from these unbalanced half-reactions sn and ag

Balance each one by adding electrons to make the charges on both sides the same:

Sn--> Sn2+ + 2 e-
Ag+ + 1 e- --> Ag

Now, you have to have the same number of electrons in the two half-reactions, so multiply the second one by 2 to get:

2 Ag+ + 2 e- --> 2 Ag

Now, just add the two half reactions together, cancelling anything that's the same on both sides:

2 Ag+ + Sn --> Sn2+ + 2 Ag

And you're done.

The mass has escaped into a gas which you can not see so the law stays true.