Let required length = x feet y inch = (12x+y) inch
he brought = y feet x inch = (12y+x) inch
y is less than 12.(because if we have 12 inch, it will be counted as 1 foot)
Since he interchanged feet and inch, he even x should be less than 12.
So, only option satisfying is x=7, y = 4
So i had asked for 7 feet 4 inch of ribbon