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100e^0.25*y = 97.5

Solving for y

Using Excel Solver

I tried using empty column entry for y in 'By changing cells' and Set objective function as LHS of above equation (empty column entry in equation included) equal to value of 97.5 in solver.

It gives no solution

How do I do this?

user5498276
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    Can you not do the arithmetic in Excel? Surely you can get `y = ...` and just run the math in Excel? (FYI `e` is `Exp(1)` in Excel). Also, it's unclear if we are to do `100e^(.25*y)` or `100 * e^.25 * y` ...it's been a while, but is that the same as `y = (log(e)(.975))/.25`? where log is "log .975, base e" – BruceWayne May 10 '16 at 17:39

3 Answers3

2

You don't really need the solver. Just re-arrange your formula to solve for Y. Since y = b^x is the same as log(b)Y = x (log of Y, with base b)

Your formula above is the same as:

Y = (log(100e)97.5))/.25

(Read aloud, that's log of 97.5, with base 100e, divided by .25

So, Y = 3.268305672

(Bonus points for someone who can tell me how to format this so the Log looks correct)

BruceWayne
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    ++ for doing the math. – Scott Craner May 10 '16 at 17:54
  • I should have specified, I know the math bit *sigh. I need to compute hundreds of these and I wanted to know how to do it on solver instead – user5498276 May 10 '16 at 19:00
  • @user5498276 - Why not just set some cells equal to each part of your formula? Like, A1 can be `100`, B1 can be `=Exp(1)`, C1 can be .25, etc. Just set up the spreadsheet like a piece of paper? – BruceWayne May 10 '16 at 19:08
  • @user5498276 - "I need to compute hundreds of these" ... does that mean you will be setting it up in VBA? – OldUgly May 11 '16 at 05:34
2

It's a bit ambiguous what you're asking...

  • Literal math interpretation: 100*(e^0.25)*y = 97.5

    Then y = 97.5 / ( 100 * exp(.25)) = .759

  • My guess of what you want: 100*e^(0.25*y) = 97.5

    Then y = ln(97.5/100) / .25 = -.101

  • Another possibility: (100 * e)^(0.25 * y) = 97.5

    Then y = (ln(97.5) / ln(100*e)) / .25 = 3.268

Whatever it is, this doesn't need solver!

Matthew Gunn
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    Agreed - however the formula should be, I think solver is overkill (and also, it robs OP of learning how to solve this, or change exponents into logs, etc....so if this is a homework question, it's certainly better to learn what to do, instead of simply throwing it into Excel). – BruceWayne May 10 '16 at 18:12
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The question is "How do I solve this exponential equation on Excel Solver?" which is a fair enough question, as it points to trying to understand how to set up solver.

My interpretation of the equation provided is given in this screenshot ...

enter image description here

The solver dialog box is then setup as follows ...

enter image description here

Of note:

  • This is a non-linear equation and needs GRG Nonlinear. If you choose LP Simplex, it will not pass the linearity test.
  • Ensure "Make Unconstrained Variables Non-Negative" is not checked.

It provided this result for me ...

enter image description here

A more precise answer can be obtained by decreasing the "Convergence" value on the GRG Non-Linear Options dialog.

A problem this simple can also be solved using Goal Seek.

enter image description here

enter image description here

OldUgly
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  • Thank you so much!! You are the only one who actually understood what I was trying to do..One question, is 5 a guess estimate? But is far away from actual value – user5498276 May 11 '16 at 15:02
  • Yes, 5 is just any number. Solver routines often work better when the starting point is far from the solution. – OldUgly May 11 '16 at 15:18
  • I used to leave that cell empty and hence never got it to work..But thanks again I didnt think of suggesting a guesstimate. I will make sure to suggest an extreme value in my other calculations with solver – user5498276 May 11 '16 at 15:28