Question 128499
Let x = number of 37-cent stamps and
    y = number of 20-cent stamps 

then y = 21-x 
then total cost = 0.37x+0.2(21-x)=7.26

then 0.37x+4.2-0.2x=7.26
then 0.17x=3.06
then x=3.06/0.17=18 
so x=18 and y = 3