Answer:

16

Step-by-step explanation:

      Mean = sum of observations / no. of observations

Let the observations are a , b , c , d and e.

 According to question :

⇒ mean of all = 10

⇒ ( a + b + c + d + e ) / 5 = 10

⇒ a + b + c + d + e = 5 * 10

⇒ a + b + c + d + e = 50    ...( 1 )    

               Mean of 1st 3 is 11 :

⇒ ( a + b + c ) / 3 = 11

⇒ a + b + c = 3 * 11 = 33     ...( 2 )

              Mean of last 3 is 11

⇒ ( c + d + e ) / 3 = 11

⇒ c + d + e = 3 * 11 = 33    ...( 3 )

            Adding ( 2 ) and ( 3 ):

⇒ ( a + b + c ) + ( c + d + e ) = 33 + 33

⇒ a + b + c + c + d + e = 66

⇒ a + b + c + d + e + c = 66

      value of a + b + c + d + e from ( 1 )

⇒ 50 + c = 66

⇒ c = 66 - 50

⇒ c = 16

⇒ third observation = c = 16