I think £320 should be £3.20.
If one sticky bun and a cup of tea cost £1.20, then 3 sticky buns and 3 teas cost 3×1.20=£3.60.
But 2 sticky buns and 3 teas cost £3.20. Therefore a sticky bun costs £0.40, because 3.60-3.20=0.40 and we have one fewer sticky bun. The tea costs 1.20-0.40=£0.80.
So there we have it: the bun costs 40p and the tea costs 80p a cup.