I'm going to assume you meant crayon instead of can:
First step is to find how many boxes she/he has left.
30 - 10 = 20
So the teacher has 20 boxes left;
Now, knowing that each box has 16 crayons, then we simply multiply the 20 by 16 to find the total number of crayons left over:
20 x 16 = 320
So the art teacher has 320 crayons left over.
Hope this helps!
