How to Calculate an unknown number?

[Guest]

I want to know how to calculate an unknown number based on facts.
I'm basically trying to perform algebraic equations with excel.
Could someone give me a hand with this theoretical question:
A man, when asked by his niece how old he was, replied, "My age is now four
times yours, but five years ago it was five times yours." How old was he?

 

[JE McGimpsey]

This just needs some algebra:

(1) y = 4x

(2) y - 5 = 5 (x - 5)

Substituting, distributing, and rearranging:

(3) y = 5 (x - 5) + 5

(4) 4x = 5x - 25 + 5

So:

(5) x = 20

and

(6) y = 80


One way, using XL:

A2: =4 * A1
A3: =5 * (A1 - 5) + 5
A4: =A3 - A2

Choose Tools/Goal Seek. Enter:

Set cell: A4
To value: 0
by changing cell: A1

and click OK.