Question 669382
First, I would convert the dimensions to inches.
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13 ft 5 in = {{{ 13*12 + 5 = 161 }}} in
15 ft 9 in = {{{ 15*12 + 9 = 180 }}} in
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Note that {{{ 161/7 = 23 }}}
and {{{ 180/7 = 25 + 5/7 }}}
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So you've got 23 tiles along one wall and
25 and 5/7 of a tile along the other wall
{{{ 23*25 = 575 }}} is the number of whole
7" tiles you need
Now you need 23 more tiles which you will 
cut 2/7 off to get the 5/7 you need
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So now you have 575 + 23 = 598 tiles
which is the number you must buy