a) Mr Smith has seven children. He wants to send three of them to run some errands on a Saturday. We will send the first one at 1pm, second one at 2pm and the third one at 4 pm. In how many ways can he choose them?
b) The children decided to actually go run all the errands together at 1pm, instead of going separately. Knowing this, in how many ways can Mr Smith select three of them to run the errands?