I asked for 9 foot 2 inches (110 inches) and my brother brought me 2 foot 9 inches (33 inches).
I asked him to buy me A feet and B inches, which is a total of 12A + B inches. Neville accidentally bought me B feet and A inches, which is 12B + A.
We know that 12B + A is actually only 30% of what we wanted, so:
12B + A = 0.3 x (12A + B)
Multiply throughout by 10:
120B + 10A = 3 x (12A + B)
120B + 10A = 36A + 3B
117B = 26A
The first number where this gives a sensible answer is when A = 9, and B = 2. QED.