First, you divide 10 by 5, which is 10/5 and then multiply it by q, which would become 10/5·q, and you don't need parentheses because using PEMDAS, you go in order from left to right. If you want to simplify it, you can make it 2q.
The minimal completion time for the activities is the shortest possible time for all the activities to be finished. In doing this, we look at the path that would require the greatest amount of time. At the START node, we choose the path that would take the longest which is 7 days leading to ACTIVITY D. Next, we choose the path leading to ACTIVITY B which takes 5 days. Then, we move to ACTIVITY C taking 5 days and finally, reach the END which would take 6 days. So, the minimal completion time is: 7 + 5 + 5 + 6 = 23 days