Grandpa tells me that the following equation is correct.


I don't get it.

Do you?

  • $\begingroup$ All of this except F is roman numerals - could that help? $\endgroup$ Commented Jan 8, 2019 at 6:41
  • 4
    $\begingroup$ I had F is RomNum for 264? $\endgroup$
    – JMP
    Commented Jan 8, 2019 at 7:09
  • 1
    $\begingroup$ May be use music notes combination with roman numerals. $\endgroup$
    – Moti
    Commented Jan 8, 2019 at 7:50
  • $\begingroup$ The Roman Numeral part is partially correct. That is a hint. $\endgroup$
    – DrD
    Commented Jan 8, 2019 at 14:28

4 Answers 4


I think the equation represents

The conversion rule between degrees Celsius and degrees Fahrenheit


Given a temperature in degrees Fahrenheit, $F$, the rule for converting to a temperature in degrees Celsius $C$ is given by $$ C = \frac{5}{9} \left(F - 32\right)$$ Multiplying both sides by $9$ we have $$ 9C = 5F - 160 $$ or $$ C \times 9 = F \times 5 - 160 $$ Converting the numbers to Roman numerals, we get $$ C \times IX = F \times V - CLX$$ or more simply $$ CIX = FV - CLX $$

  • $\begingroup$ I had thought about (rot-13) [Pavar naq Ssvir] being part of the answer, but I was stumped by [Pfvkgl]. That seems like the answer to me. $\endgroup$
    – Van
    Commented Jan 11, 2019 at 16:20


As @JonMarkPerry has already noticed we can calculate using the values of CIX and CLX as Roman numerals that F = 264, because 109 = (264 + 5) - 160.
If we represent 264 as Roman numeral it looks like CCLXIV. On the right side of the equation we have CCLXIVV - CLX. If we take out the matching letters we are left with CIVV. Now, VV that is 5 5, i.e. 10, or X and we get CIX, Q.E.D.


The initial equation you posted is: CIX = FV - CLX

If we write down the decimal value of CIX, we get

109 = FV - CLX

If we write down the decimal value of CLX as well, we get

109 = FV - 160

Take out FV

FV = 109 - 160

Add up the total

FV = 269

Take the decimal value back to roman numerals


Separate the roman numerals instead of having them in one group

FV = C + CL + X + IX

Re-order the roman numerals

FV = C + IX + CL + X

Partially add some roman numerals together


Go back to the initial formula



This shows that the equation is correct, everything except FV is given, and from getting FV it's value, we can go back to the original equation. Everything except FV is given because only FV can't be put into a decimal form with conversion from roman numerals. (only partially, but i feel like that's what makes this a trick question)


Staying with the Roman numeral concept, but focusing on it more as an equation, we can rearrange to solve for F, rather than try to make it a Roman numeral itself

Like so:

FV = CIX + CLX (add CLX to both sides, swap sides for simplicity)
FV = CX(I + L)
F = CX(I + L) / V

Substituting our Roman numerals gives:

F = 110(1 + 50) / 5

Which gives F a value of


A bit of a stretch, but...

1 + 1 + 2 + 2 = 6 and F is the 6th letter of the alphabet


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