j = jeans. S = shirts.
4s x 4j= 16
Here is another way to figure it out.
Lets say we have four shirts, A, B, C, D.
And four pairs of jeans, E, F,G,H.
A & E + A&F + A & G + A &H.
B&E+B&F+B&G+B&H
C & E + C&F + C&G+ C&H
D& E + D&F + D&G+ D&H
All of these pairs equals 16 total pairs.
I hope this helps!
