Answer:
Mai and Clare  will complete the fourth and third signs respectively at 24 minutes after beginning  the task
Mai and Noah won´t finish signs at the same time
Clare and Noah won´t finish signs at the same time
Step-by-step explanation:
A) Â During half an hour of work
Mai will finish signs at:
6 Â Â 12 Â Â 18 Â Â 24 Â Â Â Â Â 32 Â ( out of the range of 1/2 hour)
for Clare finishing time are:
8 Â Â 16 Â Â 24 Â Â Â Â Â Â Â Â 32 out
And Noah
5 Â 10 Â 15 Â 20 Â 25 Â 30 Â Â no more
Mai and Clare  will complete the fourth and third signs respectively at 24 minutes after beginning  the task
Mai and Noah won´t finish signs at the same time
Clare and Noah won´t finish signs at the same time
We can give answers to that type of question, just by examining the products of time making the sign * each different sign, as we did.
In fact, 24 is the minimum common multiple of 6 and 8
6 * 4 Â = Â 24
8 * 3 Â = Â 24
And that is the required condition to get  finished signs at the same time.