No need of a answer anymore.

mean score of class B = 1778/25 = 71.12

Step-by-step explanation:

This was your question : Class A has 12 pupils and class B has 25 pupils. Both classes sit the same maths test. The mean score for class A is 80. The mean score for both classes is 74. What is the mean score (rounded to 2 DP) in the maths test for class B?

mean of class A = Σfx/Σf

mean of class A =  80

Σfx = 80 × 12 = 960

Mean score for both classes = 74

where

b = Σfx  of class B

960 + b/37 = 74

cross multiply

960 + b = 2738

b = 2738 - 960

b = 1778

mean score of class B = Σfx/Σf

Σfx = 1778

Σf = 25

Therefore,

1778/25 = 71.12

the answer is 40.

proof:

amount of apples bought =$ 2.6

amount of peaches bought =$ 1.475

amount given by you to cashier= $ 20

amount received by me= amount given by you to cashier -   [amount of apples bought +amount of peaches bought]

                                        = 20   - [1.475 + 2.6]

                                        = 20 -   [ 4.075]

                                        =   $15.925

so, change received by me = $15.925