Answer :
I'd think you just need to either: use a timeline to add/subtract OR just find all the positive numbers (shipments and accountable books) and deduct the negative numbers (books sold).
May 1: 150
May 20: 150 - 100 = 50
June 3: 50 - 31 = 19
June 15: 19 + 25 = 44
June 17: 44 + 150 = 194
OR
150 + 25 + 150 = 325
-100 + (-31) = -131
325 - 131 = 194
May 1: 150
May 20: 150 - 100 = 50
June 3: 50 - 31 = 19
June 15: 19 + 25 = 44
June 17: 44 + 150 = 194
OR
150 + 25 + 150 = 325
-100 + (-31) = -131
325 - 131 = 194