geometry word problem

2.   

The perimeter  is 

2 ( 12 + 15)  =  2 (27)  =  54ft

The length of one diagonal  forms a hypotenuse of a right triangle  with legs of 12 and 15...its length is

sqrt  ( 12^2  + 15^2)  =   sqrt (144 + 225)  =  sqrt (289)  =  17 ft

And there are two diagonals so their combined length is  2 * 17  =  34 ft

So   .....  the total rolls she needs  is

[ 54 + 34 ]  / 20    =   88/ 20  = 4.4 rolls

But we can't purchase partial rolls...so.....she needs  5 rolls