Q-31:
20
+8
+9=37
Please note that critical path(minimum total duration) is Activity 1-- Activity 3-- Activity 4
Activity 1 will take 10 days to complete
Activity 3 will be finished on 8 day after start of Activity 2. It has F to F relationship with lead of 2 days,so it can start anytime but can only finish on 10-2 i.e 8th day.
So as of now,we have duration of CP =28
Activity 4 will be finished on 37th day. 6 days + lag of 3 days =9
Sp CP duration= 28+9 = 37
Hope it clarifies.
Q-10:
Check the formulas for free float and total float and relate it to days.
Total float= Late Start date – Early Start date or Late Finish date – Early Finish date
If you choose to start your project from day 1
Free Float= ES of next Activity – EF of current Activity - 1( Free FLoat=9 , 22,23,24,25,26,27,28,29,30)