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

Question

A = manufacturing date

B = expiry date

c = today's date

I'm using this formula to calculate the % left on today's date

((expiry date - today's date)/(expiry date - manufacturing date))*100

example

(01/01/2025 - 08/08/2021) / (01/01/2025 - 01/01/2021) = 85% 

which excel formula could I use to calculate on which day the % equals 75%?

Like this online calculator does: https://www.pccomposites.com/resources/shelf-life-calculator/

Answer 1

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. ( :

Answer 2

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.