A moving company charges $800 plus $16 per hour. Another moving company charges $720 plus $21 per hour. How long is a job that costs the same no matter which company is used.

Let x be the number of hours worked.

800 + 16x = 720 + 21x
800 - 720 = 21x - 16x
80 = 5x
x = 16
∴ A job that would cost the same would take 16 hours.

Now to double check...
800 + 16 x 16= 1056
720 + 16 x 21= 1056

The answers are correct.