(Trial and error)- well, you have to start from somewhere.........
use the 1 and the 0 to get 10.
then work backwards to find another 10 to multiply that 10 to get to 100.
so you need to play around with numbers 2,3,4,5,6,7,8,9

then

(6+4)* (8-5-2)*(7+3-9)
=
10*1*1
=10

Then the final answer (if I understood the question) might be:
100 = 1.0. * (6+4)(8-5-2)(7+3-9)

Similarly, say you decide to get to 100 from using 2*50.
Then you have to play around with numbers 1, 3, 6, 7, 8, 9 to get to 1.
Messing around with the numbers would then bring you to this:
use the 1 and 3 to get to 13.
then 1.3. - 6 - 7 + 9 - 8 = 1
so the answer would be..
100 = 2 * 5.0. *(1.3. - 6 - 7 + 9 - 8 )

Regards,

Ariella