I’m not sure if you are supposed to put an extra number and then do order of operations, but I’ll show you both with and without an extra number.
18.
With:
6 x 3 + 5
=23
Without:
6 x 3
=18
19.
With:
6 + 3(13-2) - 8
6 + 39 - 6 - 8
=31
Without:
6 + 3(13-2)
6 + 39 - 6
=39
20.
With:
17 + 15 divided by 3 - 4
17 + 5 - 4
17 + 1
=18
Without:
17 + 15 divided by 3
17 + 5
=22
Hope I helped!