NOT.5:5.3 - Choose what a backward update preserves

A plan describes two successive phases by [A:3; B:5], in whole minutes. A summary notation shows only their total, 8. The recipient changes that total to 10. Several source plans fit: [A:3; B:7] and [A:5; B:5] both do. The new total does not choose between them.

Suppose the work requires A to remain fixed. Retain A’s duration, 3, and reconstruct B as new total - 3. The requested total 10 gives [A:3; B:7]. Returning the unchanged total 8 recovers [A:3; B:5]; summing the updated plan returns 10. If instead B must remain fixed, the other update policy gives [A:5; B:5].

Assume phase durations must be nonnegative. A requested total of 2 is incompatible with retaining A at 3. Report that conflict and obtain a changed requirement if the work allows one. Substituting B = -1 would preserve the sum while violating the admitted source plan.