Well, this is what the problem tells you:
5% of c = 61
c= the total number of cans in the grocery store
so then you can do the following:
5 61
------ = ------ <----------------- here you do cross products (100*61=5c)
100 c
6100=5c <------------ you do inverse operations
6100/5=5c/5
c=1220
To check your answer you can find 5% of 1220
5 x
------ = ----- x=61
100 1220
Hope this helps :)