There are four different numbers.The average of the first three numbers is three times the fourth number,and the average of all the four number is 55.What is the average of the first three numbers?

Let the four numbers are a, b, c and d.

$$average\ =\ \frac{sum\ of\ observation\ }{number\ of\ observation}$$

Given, 

Average of first three numbers is 3 times the forth number

$$\therefore\ \ \frac{a+b+c}{3}=3d$$

$$\therefore\ \ a+b+c=9d$$............(i)

now, Average of all four number is 55. 

$$\therefore\ \frac{\ a+b+c+d}{4}\ =\ 55$$

$$\therefore\ \ a+b+c+d\ =\ 220$$.............(ii)

Putting the value of a + b + c = 9d from (i) to (ii)

we get, 

9d + d = 220

d= 22

Now, Average of first three numbers = $$3\times\ 22\ =66$$

Hence, Option D is correct.