Hello! 
We know that the length of the room is 12 1/2 ft. To find the perimeter, multiply 12 1/2 ft by 2 and subtract that from the perimeter: 
12.5 × 2 = 25 
48.5 - 25 = 23.5
Now, divide that by 2 to get the width: 
23.5 / 2 = 11.75
We can double check by using the perimeter formula: 
P = 2(12.5) + 2(11.75) 
= 48.5 
So, the width of the room is 11.75 (11 3/4) ft.