Question

A bag contains 40 balls out of which some are red, some are blue and remaining are black. If the probability of drawing a red ball is 11/20 and that of blue ball is 1/5 ,then the number of black balls is
(A) 5 (B) 25 (C) 10 (D) 30

NCERT Class X
Mathematics - Exemplar Problems

Chapter _QUESTION PAPER SET II

Answer 1

the correct answer is option c i.e 10 balls
since there are 22 red and 8 blue balls

Answer 2

Answer:

$\red {Number \: of \: black \:balls }\green {=10}$

Step-by-step explanation:

$Total \: balls \: in \: a \: bag = \orange {40}$

$Let \: number \: of \: red \:balls = \red {x}$

$Let \: number \: of \: blue \:balls = \blue {y}$

$Number \: of \: black \:balls = 40 - \red {x} - \blue {y}\:--(1)$

/* According to the problem given,

$i) Probability \: of \: drawing\: red \:ball =\frac{11}{20}$

$\implies \frac{x}{40} = \frac{11}{20}$

$\implies \red {x} = \frac{ 11\times 40}{20}= 22$

$ii) Probability \: of \: drawing\: blue \:ball =\frac{1}{5}$

$\implies \frac{y}{40} = \frac{1}{5}$

$\implies \blue {y} = \frac{ 1\times 40}{5}= 8$

$Substitute \: \red {x} \:and \: \blue {y} \: values \\in \: equation \:(1) ,we\: get$

$\red {Number \: of \: black \:balls }\\= 40 - \red {x} - \blue {y}\\=40 - 22 - 8 \\= 40 - 30\\=\green {10}$

Therefore.,

$\red {Number \: of \: black \:balls }\green {=10}$

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