The estimated times and immediate predecessors for the activities in a project at Howard Umrah's retinal scanning company are given in the following table. Assume that the activity times are independent.

$\backslash$table$[[$Activity$,$Time $($weeks$),\backslash$table$[[$Immediate$],[$Predecessor$($s$)]],$Activity,Time $($weeks$),\backslash$table$[[$Immediate$],[$Predecessor$($s$)]]],[a,m,b,a,m,b$