Answer:
490
Step-by-step explanation:
Let us first count the handshakes between two gremlins. There are 20 gremlins, so there must be 20*19/2=190 handshakes between two gremlins, making sure to divide by two to avoid over-counting.
There are 15 imps that shake hands with each of the 20 gremlins, which makes 15* 20 = 300 handshakes between imps and gremlins.
Adding these up, you have $300 + 190 = 490 total handshakes.
Hope this helps!