Math Problem help!!!

A baker was mixing light and regular butter together. The light butter contained 30% fat and the regular butter contained 70% fat. How much light butter would be required to end up with 50 pounds of a mixture that was 45% fat?

Choose one answer.

a. 31.25 pounds
b. 15 pounds
c. 18.75 pounds
d. 35 pounds

L=light butter in lbs
R=regular butter in lbs

(a) L + R = 50
(b) 0.3L + 0.7R = 0.45 (L+R)

From (a), if L + R = 50, then L = 50 - R.

Plug this into (b) and get:
0.3 (50 - R) + 0.7R = 0.45 (50-R + R)
15 - 0.3R + 0.7R = (0.45 * 50) - 0.45R + 0.45R
15 + 0.4R = 22.5
0.4R = 22.5 - 15
0.4R = 7.5
R=18.75

Plug this into (a) and get:
L + 18.75 = 50
L = 50 - 18.75
L = 31.25

Answer: a 31.25 pounds