Very Tricky 'Tube Puzzle'

THE serial number of the carriage I was riding in from Peckham Rye to Victoria recently was 72791.
It was only as the train was pulling into Victoria that I manged to solve this 'Tube Puzzle'.
As usual, each number should be used once, and once only, and must be added, subtracted, multiplied or divided to make a balanced equation.
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My solution: 7 + 1 = (9 + 7) ÷  2