With each variable having a probability of false positive of 0.05,$0.05, $ the expected value is 0.05 * number of variables = 0.05 * 100 = 5$0.05 \times\text{ number of variables} = 0.05 \times 100 = 5.$
For "at least" situations, it is generally easier to find the "other cases" and subtract from one. The probability of zero null rejections is the probability of a null NOT rejected raised to the power of the number of experiments = 0.95 ^ 3 = 0.8574. The$= 0.95 ^ 3 = 0.8574. $ The odds of at least 1$1$ is thus 1 - 0.8574 = 0.1426$1 - 0.8574 = 0.1426$ (probably no rejections).
1 - 0.95^100 = 0.9941$1 - 0.95^{100} = 0.9941$ (almost surely one or more will be rejected).
1 - 0.95^n >= 0.5$1 - 0.95^n \geq 0.5.$ This was actually just solved about an hour ago at Probability and statistical significance