Answer:
52
Explanation:
multiply the mean of 20 and it's items and the mean of 30 and it's item
20 × 40 = 800
30 × 60 = 1800
add the both result together
1800 + 800 = 2600
divide the final result by the means
2600/ (20 + 30)
2600/50
=52
∴ the mean of the whole set of items is 52
Hope this helps