Replacing weapons in Ork mob
You are allowed 1 heavy weapon per 10 Orks, but you can choose which one it is - a Big Shoota (BS) or Rokkit Launcha (RL). They don't all have to be the same type. In other words, with 30 Orks, the maximum heavy weapons you are allowed to have are either:

3RL
1BS + 2RL
2BS + 1RL
3BS