Question 1071279
mean is total/n
total=mean*n=4n pounds on Saturday.
On Sunday, the total =mean*(1/2)n=(7/2) n pounds
The total for both days was 4n+(7/2)n= 7.5 n
The number was (3/2)n
The mean is their quotient, 7.5n/(1.5n), which is 5 pounds.

Put in some numbers to check.
Suppose 100 people were there Saturday and the average was 4.  The total was 400.
On Sunday, 50 people were there and the average was 7.  The total was 350.
The overall total was 750 and the number of people 150. The average was 5.