Let's say you have a list of numbers, such as all the students' grades on a quiz. You want to find the average grade. There are two steps: First, add up all the numbers; then divide that sum by the number of numbers - that is, by the length of the list.
average uses COMBINE WITH +
Notice that the red length of block that finds the number of items in a list is different from the green length of block that finds the number of letters in a text string.

The first input to the combine with block is a two-input function. In this case, it's the + block, because we want to add all the numbers. In fact, unlike the situation with the map and keep blocks, there are really only a handful of functions you'll ever use with combine:
+, *, and, or, join, join words

Why did we include the addition and multiplication operators, but not subtraction or division? Using an operator with combine makes sense only if it doesn't matter whether the values are combined left to right or right to left. That is,
3+(4+5) = (3+4)+5
but
3-(4-5) ≠ (3-4)-5


Very occasionally you'll define a two-input custom reporter for use with combine. The two-input max block is an example; try using that to find the largest of a list of numbers.

We should try out the average block:
average of the list 7,8,1 iw 5.333333
Is that the answer you'd expect?