Paco has four pairs of jeans and four t-shirts. how many outfits of a t-shirt and a pair of jeans can paco make?

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!