Question

the average of four number is 30.if the sum of first three number is 85. the four number is

Answer 1

ANSWER WITH FULL EXPLANATION

Average of four numbers = 30
Sum of the first three numbers = 85
Let the fourth number be $x$. 
Now, all the four numbers include the three numbers (whose sum is 85) and the fourth number $x$. 
So, sum of four numbers = $85+x$

Now, we know that sum of all observations divided by the number of observations is the average of the given data. 

Now, 
Sum of all four numbers
--------------------------------    =    Average
               4
⇒ $\frac{85+x}{4}=30$
Multiplying both sides of the equation by 4. 
$\frac{85+x}{4}*4=30*4$
⇒ $85+x=120$
Subtracting 85 from both sides. 
$85+x-85=120-85$
⇒ $x=35$

∴ The fourth number is 35. 

Hope this may help you. 

If you have any doubt, then you can ask me in the comments. 

Answer 2

Step-by-step explanation:

first step

85+x

×4

4

=30×4

=85+x

=120

120-85=35

The answer is 35