The four numbers 3, 5, 7 and 9 are joined by a fifth number, and the mean of all five numbers is 7. What is the fifth number?

الشرح

Five numbers with a mean of 7 must total 35, and 3 + 5 + 7 + 9 = 24, so the fifth number is 35 - 24 = 11. Answering 7 repeats the new mean, but the first four numbers average only 6, so the joining number has to sit well above 7 in order to lift the whole set.