Current year number (say 15) and the number is down 25% vs year ago. What's last years number? formula??

Well, let's call last year's number X.

You know that this year's number is down 25 percent. That's the same as a difference of .25
Here's your formula:
% difference = (last year - this year)/last year

So, let's work it out
.25 = (x - 15)/x

.25x = x-15
-.75x = -15

And from here....

x = 20

Let original number be 100% thus:-
75%<---->15
100%<--->(100/75) x 15 = (4/3) x 15 = 4 x 5 = 20

Or, you can use the proportion:
Part/Whole = N/100 where N = the number of percents
Since it's down 25%. you're dealing with 75% of the starting number.
So:
15/W = 75/100
75W = 1500
W = 20

Well, it appears that 15 would represent 75% of previous year's number. To calculate last year's number, you would tabulate 15 / 0.75 to obtain 20.