It was a question he put to pupils when he spoke to them recently. But though they eventually reached the right answer, the chief executive is not revealing it.
Writing in Rother Bulletin, Mr Stevens says: "As part of our work in developing closer links with the schools in the district, I have taken the opportunity to visit a number of our primary schools, and spend time with the teachers and pupils, with the aim of developing closer links with the council and our work.
"I was therefore delighted to be invited to this year's leavers' ceremony for Year 6 pupils of Chantry Community Primary School.
"I felt very honoured to be asked to give a brief speech to the children and to present awards to Year 6 leavers.
"Whilst I was there ostensibly in my role as chief executive of Rother District Council, I was also there as an 'old boy' of Chantry, a school which I attended a few years ago.
"On this point, I asked the children to guess how long ago I had been a pupil at the school, and when the first one said 20 years I guess I should have stopped at that point, however, it became very much an auction and eventually they got the answer, which it would of course be unfair of me to reveal in this article.
"I had a wonderful time and thoroughly enjoyed sharing the experience with the children. the teachers and lots of parents, and whilst I would like to say that I could remember going through the same service myself, sadly the mists of time prevent a clear recollection of that time in my life!
"As a council, we have always supported the notion that the regeneration and thus the future of our district is fundamentally built on education, and it is for this reason that I and my colleagues seek out every opportunity to spend time with pupils at school, be this primary or secondary, and it is why we invite many of the schools' councils into the chamber, to talk to them about the work of the district council."