We had lots and lots of replies to this problem - it seems to have caused a great deal of interest!

There were three ways to make $99$ by putting addition signs in between the digits. John from Finningley Church of England School says:

Well, what I did was get a piece of paper and wrote down all of the possible sums. Then I added up all the sums and finally found the solutions below:
$1+23+45+6+7+8+9=99$
$12+3+4+56+7+8+9=99$
$1+2+3+4+5+67+8+9=99$

Hanna and Brian from Windsor Hill Primary School in Northern Ireland sent us this explanation of how they did the problem:

We used an approach of trial and error to help us find different solutions. We began by choosing bigger numbers to find an approximate answer and then checked the rest of the calculation to see how close we were to $99$. We continued this process adjusting our guess until we hit the target.

Many of you found all three solutions including:
Adam, Ryan, Benjamin, Rebecca, Luke, Alex, Ryo and Jake from Moorfield Junior School
Ali, Nat, Craig, Lucy, Megan, Rachel, Ryan and Toni from South Parade Junior School
Jessica, Morgan and James from Aldermaston Primary School
Khoo Kian Koon, Ong Kai Loon and Teo Mei Ting from Corporation Primary School, Singapore
Isobel, David, James, Liam, Alfie and Joe from Longwick C of E Combined School
Ã?dil from ErenkÃ? ¶y IÃ? ¾Ãƒ? ½k Elementary School in Istanbul
Charlotte and Joanna from Tattingstone School
MinJung from Hanoi International School
Prachi from Canadian Academy

Very well done. Do remember, we want to know HOW you go about solving our problems, not just your answers!