I think one null hypothesis would be that group A is more likely to complete the task than group B and then second null hypothesis to test would be that group A is more likely to complete at least some of the task. So for the first null hypothesis I think I would need to compare the proportion of each group completing the task, and then for the second I would need to compare the proportion of each group not attempting the task.

But that's as far as I can get, I'm not too sure which statistical test would be best to test these hypotheses and am not really sure what to search for to find out!

Any suggestions would be greatly appreciate.