# Ex.15.1 Q21 Probability Solution - NCERT Maths Class 10

## Question

A lot consists of \(144\) ball pens of which \(20\) are defective and the others are good. Nuri will buy a pen if it is good, but will not buy if it is defective. The shopkeeper draws one pen at random and gives it to her. What is the probability that

(i) She will buy it?

(ii) She will not buy it?

## Text Solution

**What is known?**

A lot consists of \(144\) ball pens of which \(20\) are defective and the others are good. Nuri will buy a pen if it is good but will not buy if it is defective.

**What is unknown?**

The probability that

(i) She will buy it?

(ii) She will not buy it?

**Reasoning:**

Probability of an event

\[=\frac{\begin{Bmatrix} \text { Number of}\\ \text{ possible } \\ \text{outcomes }\end{Bmatrix} }{ \begin{Bmatrix}\text { Total no of} \\ \text{favorable} \\ \text{outcomes} \end{Bmatrix} }\]

**Steps:**

Total no of ball pens } \(= 144 \)

No of defective ball pens\(= 20\)

No of good ball pens \(=144-20 =124\)

(i) Probability that Nuri will buy the pen

\[\begin{align}& =\frac{\begin{Bmatrix} \text { Number of}\\ \text{ possible } \\ \text{outcomes }\end{Bmatrix} }{ \begin{Bmatrix}\text { Total no of} \\ \text{favorable} \\ \text{outcomes} \end{Bmatrix} } \\&=\frac{124}{144} \\ &=\frac{31}{36} \end{align}\]

(ii) Probability that Nuri will not buy the pen

\[\begin{align}& =\frac{\begin{Bmatrix} \text { Number of}\\ \text{ possible } \\ \text{outcomes }\end{Bmatrix} }{ \begin{Bmatrix}\text { Total no of} \\ \text{favorable} \\ \text{outcomes} \end{Bmatrix} } \\& =\frac{20}{144} \\& =\frac{5}{36} \end{align}\]