Answer:
Part A :
Robin : 50 divided by 4 = 12.5, Robin took 12.5 Weeks to get to 50 subscribers.
Dimitri : 17 divded by 7 = 2.4, Dimitri took 2.4 weeks to get to 7 subscribers.
Part B : In 7 weeks, Dimitri will have 66 subscribers, And in 4 weeks, Robin will have 66 subscribers. So after 7 weeks for Robin and 4 for Dimitri
Part C , They will have 66 subscribers. After 7 weeks Dimitri will have 66 subcribers, and after 4 weeks Robin will have 66 subscribers.
Explanation :
I have calculated out Robins and Dimitris subscribers counts with math.