Answer

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**Hint:**To solve the above question, we use the Arithmetic progression series formula but to apply it we require first, last terms and the difference between two consecutive terms, so we will fill out all these requirements first.

**Formula used:**

\[{A_n} = a + (n - 1)d\]

**Complete step-by-step answer:**As per the question, we want to know how many two-digit numbers are divisible by 5 and leave 1 as the remainder. Which we can find out with the A.P. formula, but to apply the A.P. formula we need to find out the first, last two-digit numbers of the series, and the difference between two consecutive terms of the series.

We know that the first multiple which is divisible by 5 is 5, But it’s not a 2-digit number then We will take the second multiple of 5 is 10. It is a two-digit number and when we add 1 into it. It will become 11.

Similarly, the third multiple of 5 is 15, when we add 1 into it becomes 16.

Here, we have found the two two-digit numbers which are divisible by 5 and leave 1 as the remainder. Those are 11 and 16.

To find out the last two-digit number of the series, we will consider the last two-digit multiple of 5 that is 95, and add 1 into it, it becomes 96.

Below is the formula for A.P.

\[{A_n} = a + (n - 1)d\]

Here ${A_n}$ is the ${n^{th}}$ term of the series =96

The first term of the series: $a$ =11

The total number of terms: $n$

The difference between two consecutive terms: $d$ =5

We put all these values into the formula

$\Rightarrow$ $96 = 11 + (n - 1)5$

Simplify above equation

$\Rightarrow$ $5(n - 1) = 85$

$\Rightarrow$ $n - 1 = 17$

$\Rightarrow n = 18$

**Note:**After understanding the question we should observe that Arithmetic Progression can solve these kinds of questions too except that question where series are given and also keep remember the formula of A.P.

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