i dont get what es,ef,ls,lf

INSTRUCTION: 1. Please draw the PERT diagram for the following problem on a 8.5/11 sheet. 2. Calculate t for each of the activities t = (a + 4m+b)/6 3. Put these values of t in the diagram for each activity and now calculate ES, EF, LS, LF, Slack, and Variance for critical activities 4. Variance -[(b - a)/6] 5. What is the Critical Path? 6. What is the probability that General foundry will be able to install the air filter in the given 18 days? The Z table is given in your book on pp 579. You can use it to find the values. After finishing the problem please take pictures of the diagram and the calculations and drop these pictures in the YASSIGNMENT that read Quiz-9 BEFORE 3.30 P.M. General Foundry, Inc., a metalworks plant in Boston, has long been trying to avoid the expense of installing air pollution control equipment. The local environmental protection group has recently given the foundry 18 days to install a complex air filter system on its main smokestack. General foundry was warned that it will be forced to close unless the device is installed in the allotted period. Following activities need to be completed for the installation of the air filter system. What is the probability that General foundry will be able to install the air filter in the given 18 days? Activity m b + ES EF LS LP Slack Variance Immediate predecessor 1 2 2 3 B 2 3 4 B 1 2 3 D 2 2 4 6 E CD 1 4 7 F B 1 2 9 G EF 3 3 4 11 H CD 1 2 3