A penny collection contains twelve 1976 pennies, seven 1968 pennies and eleven 1971 pennies. If you are to pick some pennies without looking at the dates, what is the least number that you must pick to be sure of getting at least five pennies from the same year?

My working:

I tried the generalized principle: N(X)>k.N(Y) where N(X) defines the number of pennies. So it should be 30>6.5 --> But the solution says 13. Am i applying the principle wrongly?