Crashing a project

[rowdy3]

By how many days should each activity be reduced the project completion time by 4 days? I have all the parts right except for part B. I thought it was 2 but it's not. What I did was subtract normal time from crash time and it worked for all the problems except B. Here's a link to the table. Any ideas?

 

[Ishuda]

My take on the situation:

First of all consider that A must be finished before D can start and C must be finished before E can start. Thus total Normal time is the minimum of (A+D), B, and (C+E) which is 13 days. So you want to reduce times to make this 9 days:
A+D \(\displaystyle \le\) 9
B \(\displaystyle \le\) 9
C+E \(\displaystyle \le\) 9
with conditions given, i.e. crash A before D because it's cheaper, maximum crash for A is 1 day, ....
Do you really need to reduce B?

Your method worked because the crash times for A and D added to 9 and the crash times for C and E added to 9. In other cases it might not be the case that the given crash times add to the wanted completion time. For example what if it were wanted to complete the project in 10 days. The reduction would be {A=1, B=0, C=1, D=1, E=2}