Scientific Notation big equation?

Question

okay i have this multiple choice question and i don't know the answer. here's the exact question:

"Voyager 1 has been averaging a speed of about 15 km/s. At this rate of speed, how long (in hours) will it take to arrive to the nearest star Proxima Centauri, which is 3.99 x 10^13 km away from Earth?

A. 6.55 x 10^8 hours

B. 7.39 x 10^8 hours

C. 8.12 x 10^8 hours

D. 9.21 x 10^8 hours

Answer 1

(3.99 * 10^13 km) / [ (15 km / sec)(3600 sec / hr) ]

= (3.99 * 10^13) / (15 * 3600) hr

= 7.39 * 10^8 hr.

Answer B.