Maths lesson

The engine that failed on Tuesday was one of 85 Rolls-Royce RB211 engines in the Qantas fleet. They are used on 19 other of the airline’s Boeing 747s.

via theage.com.au

A B747 has four engines. Why is there an odd number of engines in the Qantas fleet? Unless there’s another 747 running around with a spare stuck into the tail, DC-10 style.

Qantas says they have 19 “other” 747s, so that’s 20 total. Doing the maths, 20 x 4 = 80. So that’s five engines unaccounted for. Are there five of these engines whizzing about, unattached to aeroplanes?