Question

The mean of $100$ observations were calculated as $40$ by a student who took by mistake $50$ instead of $40$ for one observation. What are the correct mean ?

Answer 1

The no.of observations are $100$

Mean =$40$

Sum of observations $40\times 100=4000$

wrong observation $50$

Sum of observations $4000-50=3950$

Right observation is $40$

Sum of observations $3950+40=3990$

Mean is $\frac{3990}{100}=39.9$