Question 1109127
Arithmetic Mean= Sum of all observations/No. of observations
As given in question 
  Mean= 30
  No. of observations= 5 
So 30= Sum of all observations/5
   30x5= sum of all observations
   sum of all observations = 150
As given 
 values of all observations are 18,23,28,x,43
We have to find x
 Sum of all observations = 18+23+28+x+43
  150=112+x
  150-112=x
  x= 38