a fair coin is flipped four times. What is the probability of getting tails at least twice?

Flipping 4 times the probability of

getting 0 tails: (\frac{1}{2})^4=\frac{1}{16}
getting 1 tail: 4(\frac{1}{2})^4=\frac{1}{4}

Thus the probability of getting at least 2 tails is 1-\frac{1}{16}-\frac{1}{4}.