A pavement was tiled by 80 parallel slabs of tiles. A cat walked along the pavement and at every fifth tile (other than the first) it relieved itself, i.e. at tiles numbered 5, 10, 15, ... A while later, another cat came along and did the same thing but only at every seventh tile (other than the first), i.e. at tiles numbered 7, 14, 21, ... Later a boy strolled along the pavement, each step covering the odd-numbered tiles. How many of the tiles covered by his steps will not have been soiled by the two cats? Dr Leong Yu Kiang

if you cant solve it in 5 simple steps, you might want to go for counting classes.