Formula to calculate progress

Hello all,

I am trying to write a formula that calculates, the completed progress after each stage of a task. I.E.

Column A is Starting Amount, Column B is Balance at 10am, Column C is balance at 2pm and Column D is balance at 4pm. Column E is Progress to date (time?)

SO, at 7am, I have 100 apples to pick, at 10am, I still have 75 of them left to pick, which means my progress in (Col E) should read 25 apples.
Next check at 2pm, I still have 45 left to pick, and therefore my progress in Col E should now read 55 apples, and finally at 4pm I have 2 apples left
to pick, so my total progress in Col E should now read 98 (apples).

At each stage I want to track my progress in column E. How do I write a formula for this?

Try:

=A2-IFERROR(LOOKUP(2,1/(B2:D2<>""),B2:D2),0)

You're welcome & thanks for the feedback.