Instructions Father Christmas is delivering cards to houses numbered 1 to 100. The price for each card is the same as the house number so, for example, house number 9 requires 9p in stamps, house number 38 needs 38p worth of stamps and of course, house number 39 needs 39p. Unfortunately, his elves have run out of all the stamps except for 3p and 5p. The investigation is to find out which house numbers will get cards and which won't. The children will find that 9p can be made with three 3p stamps, 10p can be made with two 5p stamps, 11p can be made with two 3p and one 5p stamp etc. Obviously house number 1 will not get any cards as it's not possible to produce 1p with 3p and 5p stamps.