Explanation:

Let's look how much does the party cost for some numbers of children:

1. For 1 child the price would be $40 + 4 = 44$
2. For 2 children $40 + 8 = 48$
3. For $n$ children $40 + 4 n$

The formula in point 3. is a general formula to calculate cost of the party for $n$ children, so we can use it in our task.

We have to calculate how many children can take part in a party for $76. So we have to solve inequality: $40 + 4 n \le 76$$4 n \le 76 - 40$$4 n \le 36$$n \le 9\$