Mean of 10 observations is 20 and that of other 15 observations is 16. Mean of all 25 observations will be :

1. 16.6

2. 18.6

3. 19.6

4. 17.6

Mean of 10 observations = 20

Mean of 15 observations = 16

Mean = $\dfrac{\text{Total sum}}{\text{Total number of observations}}$

Sum of 10 observations = 10 × 20 = 200

Sum of 15 observations = 15 × 16 = 240

Total Sum = 200 + 240 = 440

Total number of observations = 25

Mean = $\dfrac{440}{25}$ = 17.6.

Hence, option 4 is the correct option.