What is the value of r, the part of the job that Marina can complete in 1 hour? 0.1 0.4 0.5 0.6

So, Katherine can do 0.1 of the job per hour. (if she needs 10 hours for the job, each hour she can do the whole divided by the time, so 1/10, which is 0.1)

if in two hours she and Marina can be done, then Marina will do 1 (the whole job) - 2/10 (twice as much as she can do in an hour) = 0.8 of the job. So if Marina does 0.8 of the job in two hours, each hour she will do half of it: 0.4 - and this is the correct answer

The correct answer to this is 0.4 i just took the test, its right