Marcus mixes 5 parts white paint with 2 parts red paint to make pink. Suppose he adds one part of each. What is the new ratio of red to white?

The original ratio  is 5/2.
If Marcus adds 1 part of each to the mixture, the new ratio of red to white becomes 

5+1         6       2
------ = ----- =  ----
2+1         3       1

The new ratio of red to white is 2:1.