This proved a very interesting question and again you gave us some excellent answers. Ashley, Edward, Jake and Adam of Middlewich Primary School say:

19 is our special number.

1+9=10
1x9=9
10+9=19

They used trial and error.

However, Annabel who is 10 years old and goes to Cottesmore School sent us a different solution:

4x9=36
4+9=13
36+13=49

The answer is 49

But it doesn't end there! Tom, Hussam and Suzy in Year 5 at Carleton St. Hilda C.E Primary School have another version:

The sum of 99 is 18
The product of 99 is 81

Therefore if you add 18 to 81 you will end up with the number 99

Just when you thought that was all, Charlotte, also in Year 5, at Woodfall Junior School gives us something extra to ponder on:

I found (1 + 9) + ( 1 X 9) =19
and (2 + 9 ) + ( 2 X 9 ) = 29

I predicted that it will work for any two digit number with 9 in the units column

. I tested it for 39, 49, 59 and 99.

This is definitely worth exploring, well done Charlotte. Has anyone tested the other two digit numbers with a 9 in the units column? Can you explain why this works?