Maria is ordering copy paper for her office. The office is low on white paper and on blue paper. She will pay $3.50 per realm of white paper and $3.75 per realm of blue paper. If the purchase order is for $36.00 and she ordered 10 realms of paper, how many realms of white paper will she receive

You have:

 x: the number of reams of white paper purchased 
 y:the number of reams of blue paper purchased 

 Then, you have the following system of equations:

 x+y=10
 3.50x+3.75y=36

 When you solve the system shown above, you obtain:

 x=6
 y=4

 The answer is: 6 reams of white paper