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 ?
"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 ."
inverse(add(divide(const_1, 5), divide(const_1, 8)))