To find the mean mark the formula is mean = ∑marks /number of students Therefore the mean for maths is 90 and so ∑marks = meannumber of students = 905 = 450 and the ∑marks for the physics class is 85*10= 850. Therefore total number of marks is 450 + 850 = 1300. Then the mean of both sets of data is the total number of marks divided by total number of students. Total number of students = 10+5 = 15; therefore the mean of both sets of data is 1300/15 = 86.7 (to 3 s.f.)