Let the total number of pies be x. 

(x/5) + 30 = (x/3)

Solving for x: 

x = 225

(2/3)x = 150 (answer for part 1)

Let the number of boxes that contain 6 pies be a and the number of boxes that contain 12 pies be b. 

6a + 12b = 150   –(1)

a + b = 18 

b = 18 – a  –(2)

Subst. (2) into (1)

6a + 12(18-a) = 150

Solving for a: 

a = 11

There are 11 boxes that contain 6 pies.