The average weight of the 5-man freshman basketball team was thought to be 60 kg. However, a mistake was made — one of the weights was written down wrong. Once the mistake was found, and that one weight was changed to the right value of 80 kg, the correct average turned out to be 70 kg. What was the mistaken value? That is, what was written down at first instead of 80 kg?
If the average was thought to be 60 kg, the sum of the five weights was 300 kg. If the average was really 70 kg, the corrected sum was 350 kg. Thus the first sum was 50 less, so the 80 kg must have been written as 30 kg.