Question

A man receives Rs. 60 for the first week and Rs. 3 more each week than the preceeding week. How much does he earns by the 20th week​

Answer 1

The man's income by the 20th week will be Rs. 1770

Explanation:

Given that the income of man in first week is Rs. 60

It is also mentioned that the income increases every week then the previous one.

which means the weekly incomes will be like:

60,63,66,69.....

As the common difference of terms is same i.e. 3, it forms an arithmetic sequence.

Where

`a_1 = 60\\d = 3`

The general formula for arithmetic sequence is:

`a_n = a_1+(n-1)d`

Putting the values of d and a_1 we get

`a_n = 60+(n-1)(3)\\a_n = 60+3n-3\\a_n = 57+3n`

This is the general formula to find the income of any week. Now for 20th week,

n = 20

`a_(20) = 57+(20)(3)\\a_(20) = 57+60\\a_(20) = 117`

The income for 20th week is 117.

As we have to find the total income for 20 weeks

We will use the summation formula for arithmetic sequence.

`S_n = (n)/(2)(a_1+a_(20)})`

Putting the values

`S_n = (20)/(2)(60+117}\\S_n = 10(177}\\S_n = 1770\ Rs.`

Hence,

The man's income by the 20th week will be Rs. 1770