Question

What is the unit digit of sum of the first 78 whole numbers

Answer 1

Answer:

1

Step-by-step explanation:

sum=(n(n+1))/2

sum=(78*79)/2

sum=3081

unit digit=1

Answer 2

Answer:

Unit digit = 3

Step-by-step explanation:

In the given question,

Total numbers that we have = 78

All of these numbers are whole numbers starting from 0 and consecutive henceforth.

So,

Also,

We know that the sum of the AP is given by,

$S=\frac{n}{2}(2a+(n-1)d)$

where,

n is the number of terms

a is the first term

and,

d is the common difference.

Now,

a = 0

d = 1 ( ∵ consecutive numbers)

n = 78

On putting these values we get,

$S=\frac{78}{2}(2(0)+(78-1)1)\\S=39(77)\\S=3003$

Therefore, the sum of the first 78 whole numbers is 3003.

Therefore. the unit digit of the sum is 3.