Robin has 28 packages of gum and 13 packages of candy. There are 4 pieces in each package. How many pieces of gum does Robin have?
Let's denote the number of packages of gum by g and the number of packages of candy by c. Then we have 

g+c=28 
and 
c=13. 

Therefore 

g=28-13=15. 

Now we need to calculate the number of pieces of gum. We know that there are 4 pieces in each package. Therefore the number of pieces of gum is 

4*15=60.