Allah messed up with calculations.. again..

[muhtesem insan]

My point is that the teacher is showing his mercy to the students by making the task easier for them. He doesn't have to be omniscient to do that, does he?

But teacher may be mistaken so he can correct his mistake, do you say allah was mistaken at first?

Allah does know that it would be too heavy for them. He purposely made it difficult at first, so that later, He could reduce the number as a way of displaying His mercy toward the Muslims. That is essentially why He "lightened their burden," which is, to manifest His mercy to them only after giving them a difficult challenge. This is similar to how Allah reduced the number of daily prayers from 50 prayers to 5 prayers when Prophet Muhammad (pbuh) asked Allah to reduce the number because he knows that performing 50 prayers a day would be too much of a difficult thing for his followers. Then, Allah showed His mercy by reducing it to 5 prayers only and thus He made the task lighter for the Muslims.

So allah showed mercy by lightening a burden he caused in the first place.