Hi Mikk,

I am not sure if I understood the question correctly.

**First interpretation of question:**

in Round 1, the person called 4 people and in the next rounds, the 4 people will call another 4 people each, and so on.

If that is the case, number of calls in:

round 5 is 4^{5}

round 4 is 4^{4}

round 3 is 4^{3}

round 2 is 4^{2 and}

round 1 is 4.

Note that this is a geometric progression. So when we find the sum, we are finding the geometric series. The formula to find geometric series up to n terms is:

(a(1-r^{n}))/(1-r),

where a is the first term and r is the common ratio of the sequence.

So when you add them up:

4+4^{2}+4^{3}+4^{4}+4^{5} .

first term, a = 4,

common ratio = 4

number of terms = 5

Applying the formula:

4+4^{2}+4^{3}+4^{4}+4^{5}

= (4(1-4^{5}))/(1-4)

=1364

The **second interpretation** of the question could be one person called 4 people in round 1 and in round 2 he called 4 more people, which means he called 8 people in round 2, and in round, he called 4 more people than the previous round.

If this is the case, number of calls in

round 1: 4

round 2: 8 i.e. 2*4

round 3: 3*4

round 4: 4*4

round 5: 5*4 = 20

In this case, this is an arithmetic progression. So if we add them up, it will be an arithmetic series. To add them up to n terms, the formula is

(n/2)(a+l) or (n/2)(2a+(n-1)d)

where a is the first term, l is the last term of the series and d is the common difference.

For this question:

total number of calls by round 5

= (5/2)(4+20)

= 60

the other formula..

total number of calls by round 5

=(5/2)(2*4 + (5-1)4)

=60

Let me know if i interpreted the question correctly.