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### Question

The average weight of a class of 30 students is 13 Kg, and the average weight of a class of 60 students is 7 Kg. What is the average weight of the two combined classes?

**A**

9 Kg.

**B**

10 Kg.

**C**

8 Kg.

**D**

12 Kg.

**Soln.**

**Ans: a**

If a sample n_{1} has a weighted average of A_{1}, and another sample n_{2} has a weighted average of A_{2}, then the weighted average, A,of the combined samples is determined by the alligation formula as: n_{1}(A - A_{1}) = n_{2}(A_{2} - A). Putting n_{1} = 30, A_{1} = 13, n_{2} = 60 and A_{2} = 7, we get A = 9 Kg.