Customer account "numbers" for a certain company consists of 3 letters followed by 2 single-digit numbers. How many different account numbers are possible if repetitions of letters and digits are allowed?

Initially, 10 people have an infection. The next day, every infected people infects one moreperson. If this pattern continues, how many days will it take for 1280 people to be infected?

Suppose Allan is going to build a playlist that contains 7 songs. In how many ways can Allan arrange the 7 songs on the playlist? Allan can arrange the 7 songs on the playlist in different ways.

In a school there are 20 teachers who teach mathematics or physics. Of these, 12 teach mathematics and 4 teach both physics and mathematics. How many teach physics ?