At a fast food restaurant, 10 cheeseburgers w/onion and 5 orders of french fries contain 6900 calories. 5 cheeseburgers w/onion and 3 orders of french fries contain 3660 calories. How many calories are in a cheeseburger w/onion? How many calories are in one order of french fries?

Write the equations,

10 c + 5 f = 6900

5c + 3f = 3660

Double,

10c + 6f = 7320

Subtract,

f = 7320 - 6900 = 420

10 c = 6900 - 5f = 6900 - 2100 = 4800

c = 480

Answer: Cheeseburger 480 calories, fries 420 calories

Check:

10 (480) + 5 (420) = 4800 + 2100 = 6900 good

5 (480) + 3 (420) = 2400 + 1260 = 3660 good