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9 = si / so
9 so = si
1/f = 1/si + 1/so
1/4 = 1/9so + 1/so
1/4 = (1 + 9) / 9 so
1/4 = 10 / 9 so
40 = 9 so
40 / 9 = so
9 so = si
9 . 40/9 = si
40 cm = si