A worker can load one truck in 5 hours. A second worker can load the same truck in 4 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?  Select:
A) 1.8
B) 2.0
C) 2.2
D) 2.4
E) 2.6

Rationale: "the workers fill the truck at a rate of 1 / 5 + 1 / 4 = 9 / 20 of the truck per hour . then the time to fill one truck is 20 / 9 which is about 2.2 hours . the answer is c ."
Answer: C