Answer 1

We have,
The mean of 5 observations is 50.
⇒ The total sum of 5 observations = 50 × 5 = 250

Also, the mean of 4 observations is 45.
⇒ The total sum of 4 observations = 45 × 4 = 180

So, the observation which was removed  = 250 − 180 = 70.