Part of an email I got from my good old friend Jyotsna from Singapore:

“Btw your puzzle format on FB was an inspiration to the math teachers at the boy’s school. I had asked the teachers to look at posting puzzles in a forum so that kids can help out each other and gave your puzzles discussion as an example. It is the second week of this discussion on the school forum and the class kids seem to love it!! So you are pretty influential in far away Singapore! :-)”

What started as an exercise to get rid of boredom on a flight back from DC a few years back has certainly had some unintentional – but highly welcome 🙂 – consequences. Thank you Jyotsna!

This puts pressure on me to post the next puzzle. This one is dedicated to the kids in Singapore and elsewhere. And also adults.

(Again, do not post in Comment section; send me FB msg; I will respond)

A few days back, I was cleaning my pool and talking to my another good friend Kuntal over the phone. He had posed a question – “what is the last digit of 19^19 + 99^99”. Over the phone we discussed our answers – he had taken an elegant way to solve it – I had used a short cut.

So let’s try that:

1. What is the last digit of the sum of 19 to the power of 19 added to 99 to the power of 99. (19^19 + 99^99).
And an extension..
2. What is the last digit of
9^9 + 19^19 + 29^29 + 39^39 +……+ 99^99

3. If you have an younger kid – or you are stuck yourself – try with the following simpler version – and then the above two become easier. What is the last digit of 99^99?