You would need to get 40 ties because if each tile were a full feet then you would need 20 but since it is each a half a foot then you would need 40 tiles because you have to double it. 2 tiles equals a foot long tile.
7 I think because if you would put a zero behind the 4 and 7 and add them together it will be 110 and all that's left if u subtract them from 180 is 70 (7)