Re: Spoilers!
All you need from memory is the boiling and freezing pts in both F and C.
We know there are 100 degrees between freezing and boiling in C,
212-32 = 180 degrees in F. So the C degree is bigger than the F by 180/100 = 9/5.
But freezing is 32 in F, so F = 9/5C + 32. You can check by letting C = 100
and F becomes 212 which we already know.
If we call the number where F and C are equal ' T', then, T = 9/5T + 32.
Simplifying and solving for T, -4/5T = 32 or T = - 40.
Of course, it took me a lot longer to figure that out than it did to write it down. In fact, it took me a great deal longer to figure it out than it should have.