2) Know what to do but can't get answer, as I'm not familiar with the imperial measurement system

5) There are 5 candles, and the number of combinations of 3 candles is given by which gives 10.

7) We want a number between 1900 and 2000 that is a multiple of 7 and 13. Start with 7x13 = 91. Times by 20 = 1820.

So we know 1820 is divisible by 7 and 13. We also know 91 is. So 1820 + 91 = 1911. This is the answer we want.