It means that there are two different types of tasks available. What you would do for the task for $0.24 would be different than what you would do for the one for $0.20 (most likely).
When they say that there are '151 available', it means that there are 151 different opportunities to earn $0.24 for that specific task. So, if you did two sets of those, you'd earn $0.48 and there'd be 149 left for you or others to do.
Hope that made sense.