Answer:
63
Step-by-step explanation:
Mean= All the scores added together/number of pupils. You first have to find what the scores added together are, so you'll take the mean score of A TIMES the number of pupils in A to find all the scores added together of A then repeat with B. You then get 30*78= 2340 for A and 25*45= 1125 for B. Add up 2340+1125 to get 3465. You then divide 3465 (all the scores added together) by 55 (the total amount of pupils) to get 63 (your answer).
Hope this answer helped! :)