Microsoft Excel. Shelf life: calculate on which day % left equals 75

Let :

a = expiry date
b = some date
c = manufacturing date

Then :

 (a - b)/(a - c)= 75% = 0.75

Rearrange :

 (a - b) = 0.75 * (a - c)
 - b = (0.75*(a - c)) - a

 b = a - (0.75*(a - c))

So your formula should be :

 some date = expiry date - (0.75*(expiry date - manufacturing date))

Please share if it works/understandable/not. ( :

In my opinion, you may simplify the formula for finding the target date first.

For example, A = manufacturing date, B = expiry date, C = Date at 75% remaining.

Then you could get as following:

(B-C)/(B-A)=0.75

C=0.25*B+0.75A

Then you may use the formula =0.25*B2+0.75*A2 in C2 on my sample, please remember to set the cells with results as Date.