What are the possible values of a 100-ohm resistor with a 10% tolerance?

Tolerance indicates the acceptable percentage variance from the stated (nominal) value of the resistor. To find the allowed range, you must calculate the percentage of the stated value and both add it to and subtract it from the nominal resistance. For a 100-ohm resistor, a 10% tolerance means the actual resistance can vary by 10% of 100 ohms. Since 10% of 100 is 10, the variance is +/- 10 ohms. Subtracting 10 from 100 gives the lower limit of 90 ohms, and adding 10 gives the upper limit of 110 ohms. Therefore, any value between 90 ohms and 110 ohms is within spec.