We need to use numbers 1-9 to equal 100. You can use any arithmetic operation you'd like (+, -, /, *). To achieve an answer you would have to play around with what arithmetic operation you use for each pair of numbers however this seems to do the trick:

9*8+(7+6+5+4+3+2+1)

I put the parentheses in so that you can see how this math turns out:

72 + (28) = 100

And we have a 100. Just multiply 9*8 and add the rest of the numbers and you'll get 100.

Did that help? If so could you choose my answer as the best answer please? Thank you!