Hi David,
The nominal resistance is value of the label resistance. In this case, it's 100 Ω. Since there are five 100 Ω resistors in series, it's the sum of the five resistors. Therefore the total nominal resistance is 500 Ω.
Now for the tolerance, on one hand we have +5% and on the other hand we have -5%. For a resistor with +5% tolerance, this means that the actual resistance is 5% above the labeled value and vice versa for -5%. In real terms for +5% tolerance the actual value of a resistor is 105Ω. And for a -5% tolerance the actual resistor value is 95Ω.
Knowing that lets look at the bounds. If all 5 resistors have a -5% tolerance then that is the product of 5*95 = 475Ω. For the +5% tolerance, the total resistance is the product of 5*105 = 525Ω.
To put this into statistics terms, the total nominal value or the mean of five 100Ω resistors in series is 500Ω. For your lower bounds, the total is 475Ω and for the upper bounds is 525Ω.
