Standard deviation of lifespan

Question

Within a population of 70 flies, the average lifespan is two months. 60 of the flies live an average of 1.5 months. What is the average standard deviation, in months, of the lifespan of the remaining flies?

Answer 1

Average = sum/N 

 

 

The average of 70 of them is 2:    Grand total / 70 = 2

 

                             So the grand total is 140.

 

 

The average of 60 of them is 1.5:     Partial sum / 60 = 1.5

 

                                              So the partial sum = 1.5 * 60 = 90

 

  The remaining total is 140 - 90 = 50

 

The average is 50/10 = 5 months