To calculate the mean (average) of a set of numbers, follow these steps:

1. Add all the numbers: $87 + 79 + (-3) + 15 + 43 = 221$

2. Count how many numbers there are: There are 5 numbers.

3. Divide the sum by the number of values: $\frac{221}{5} = 44.2$

Thus, the mean is 44.2.

Would you like more details or have any questions?

Here are five related questions for further practice:

1. What is the median of the numbers 87, 79, -3, 15, and 43?
2. How do you find the mode of a set of numbers, and is there one in this set?
3. What is the range of the numbers in this set?
4. How do you calculate the mean if the set has a negative number?
5. What is the effect of adding another number to the set on the mean?

Tip: When calculating the mean, always ensure to consider both positive and negative numbers properly to avoid mistakes.