Cedric can weld 200 flanges in 18 hours. It takes Mike 20 hours to weld 200 flanges. If they work together, how long will it take them to weld 200 flanges?

Answer 1

1/18 + 1/20 + 1/x

multiply by 180x

10x + 9x = 180

19x = 180

x = 180/19 = about 9.47 hours to do 200 together

If they had each done 200 in 19 hours

they'd do it together in 19/2 = 9.5 hours

but it's slightly less if they average 19 hours, but each close to 19

the faster guy reduces the average 9.5 down to 9.47 hours