Why is it that for ex10 part b i believe the answer is 40?

One card is dealt from a 52 card deck. find the probability that the dealt card is a 4 or a black 7

On average, how many more hits will he get when he bats 100 times at home during the day as opposed to 100 times away at night?

On a string instrument, the length of a string varies inversely as the frequency of its vibrations. an 11-inch string has a frequency of 400 cylces per second. find the frequency of a 10-icnch string.