Hello. I came across the above sum, the other day, in one of those Facebook puzzle posts. I don't normally look at this sort of thing, but I had a quick glance at this one and am still trying to get to the bottom of things. The way I was taught, the answer to this sum is 9. But there appears to be another school of thought where the answer is 29. At first, I thought it was a wind up, but it seems there are some genuine people, teachers even, who insist the answer is 29. Can someone please explain how there can be two schools of thought to solving this sum. If I do the calculation on one of my personal calculators or on the Windows 10 calculator, the answer is 9. Obviously!! But if I do the calculation on my mobile phone, the answer is in fact 29!! How can this be? I find this very worrying. Can someone please explain how this can be and how long ago the second method came about and which method came first and who, on earth, is right etc etc. Many thanks.