L = large pumpkin
m = medium pumpkin
Write a system of equations:
1) L + m = 20
2) m = L - 6
3) $3L + $2m = $53
Substitute L - 6 for m in equation 1:
L + (L - 6) = 20
Add 6 to both sides:
2L = 26
(L + L = 2L)
Divide two on both sides:
L = 13
Julie bought 13 large pumpkins.
Check (use equation 3):
$3(13) + $2(7) = 53
(m = 7 because 20 - 13 = 7)
Multiply:
$39 + $14 = $53 :)