A worker can load one truck in 5 hours. A second worker can load the same truck in 8 hours. If both workers load one truck simultaneously while maintaining their constant rates, approximately how long, in hours, will it take them to fill one truck?
Choose the correct option:
A) 2.5
B) 2.7
C) 2.9
D) 3.1
E) 3.3

Rationale: "the workers fill the truck at a rate of 1 / 5 + 1 / 8 = 13 / 40 of the truck per hour . then the time to fill one truck is 40 / 13 which is about 3.1 hours . the answer is d ."
Answer: D