If you write plus signs between each of the digits 1 to 9, this is what you get:

1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 = 45

However, if you alter where the plus signs go, you could also get:

12 + 3 + 45 + 6 + 7 + 8 + 9 = 90

Can you put plus signs in so this is true?

1   2  3  4  5  6  7  8  9  = 99