if

A1=54 (TOTAL OF M1typeBuses)
A2=24 (TOTAL of M2typeBuses)
and

A3=78(A1+A2)


How to structure the formula, so that I can change values of A1/A2/A3 ,
so that that they interchange, based on the total Value of A3.


In other words,
if A1 changes to 50,
then A2=78-50=28


But if I change A3 to 80 for example,
the formula should ak the user where he want
these extra 2 buses be added to:
to A1 cell,
or A2 cell


p.s. it is probably fairly simple issue for some of you,
but I have tried to do it with sumif and sumifs
and ifs... It is out of my level of understanding excel formulas.
10X in Advance