I’m looking at a linear trend line [....]
So day 6’s value = 6
Day 20’s value = 20
How can I work out day 25? Based on the trend line from the two values above? The answer will be 25
Use the FORECAST function. But you have provided very different data; the first has 3 pairs of values, not just 2. So I'm confused.
Suppose that A1:A6 contain the day numbers 1,3,5,6,20,25. And B1:B5 contain the corresponding values that you posted, namely: 1,5,10,6(!),20.
Then the estimated values might be calculated in one of 3 ways, depending on your intent (which I'm confused about).
1. Based on the original 3 data pairs. Enter the following into C1 and copy down through C6:
=FORECAST(A1,$B$1:$B$3,$A$1:$A$3)
2. Based on 5 data pairs: the original 3 and the additional 2. Enter the following into D1 and copy down through D6:
=FORECAST(A1,$B$1:$B$5,$A$1:$A$5)
3. Based on just the last 2 data pairs. Enter the following into E1 and copy down through E6:
=FORECAST(A1,$B$4:$B$5,$A$4:$A$5)