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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?
$1\;$ $2\;$ $3\;$ $4\;$ $5\;$ $6\;$ $7\;$ $8\;$ $9\;$ $= 99$
How many ways can you do it?