Because the question says that the recent dividend of 50c is 10c more than the dividend 3 years ago. Therefore the dividend 3 years ago was 10c less the 50c, which is 40c.
Mr Moffat, thank you for this clarification. I understood it differently. .10c more than three years ago as “more than 3 years ago” – like 4 years or 5 years and so on.