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

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

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