Question

What is the value of the sum 5 + 10 + 15 + ... + 95 + 100? ...?

Answer 1

5+10+15+20+.....+100
=5(1+2+3+..+20)
=5{ 20(20+1)/2} [sum of n natural numbers =n(n+1)/2
=5{210}
=1050
    

Answer 2

 First the number of natural numbers up till 100. 

Using this formula: Sum to n = (n(n+1))/2 
So, for you n = 100 

Therefore, (100x101)/2 = 5050 keep this as value 'x' 

Second, the numbers divisible by 5. 
Numbers divisible by 5 from 1 to a 100 are: 
5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 and 100 

Add all these up = 1050 remember this as value 'y' 

Now, minus x from y which is 5050 - 1050 = 4000 


Hope this helps.