Graph Theory Coloring

Question

There are some earthlings and 15 martians in a space shuttle. Each earth- ling shook hands with exactly 6 martians, and each martian shook hands with exactly 8 earthlings. How many earthlings are there in the shuttle?

Tried a picture, but it got messy. There should be a formula to make this way easy, but I can't find it.

Answer 1

The total number of handshakes done by all the earthlings = total number of handshakes done by all the martians. The total number of handshakes done by the martians = $8\times15$ Therefore if the total number of earthlings is $x$, then $6\times x=8\times 15$. That is, there were $x=20$ earthlings.

Answer 2

Note that if each earthling shook hands with $6$ martians, and each martian shook hands with $8$ earthlings, then the ratio of martians to earthlings must be $6/8 = 3/4$, so there must be $\frac 4 3 \cdot 15 = 20$ earthlings.