Kevin can do a job in 4 hours thus his rate is 1/4 job per hour

It takes Kendra 2 hours to do the same job this his rate is 1/2 job per hour

let x = the hours they need to do the job together thus their together rate is 1/xjob per hour

1/4 + 1/2 = 1/x

3/4 = 1/x

x = 4/3 hr

x = 1 hr 60/3 min

x = 1 hr 20 min

it will take them 1 hour and 20 min to do the job together.