In cell A1 put 100
In cell A2 put 5
In cell A3 put =A1*A2 and it will show 500
With cell A3 selected, do Tools / Goalseek. It should already say
Set Cell < A3 >
To Value < > <----- Put say 300 in here
By changing cell < > <----- Use selection button to click onto cell A2.
Hit OK and Excel will work out what value needs to go into cell A2 to give you a
final total of 300 in cell A3. It will then overwrite what is in cell A2 with
the value it has calculated.