Question 12 - CBSE Class 10 Sample Paper for 2018 Boards

Last updated at Sept. 14, 2018 by Teachoo

A box contains 12 balls of which some are red in colour. If 6 more red balls are put in the box and a ball is drawn at random, the probability of drawing a red ball doubles than what it was before. Find the number of red balls in the bag.

This is a question of CBSE Sample Paper - Class 10 - 2017/18.

Question 12 A box contains 12 balls of which some are red in colour. If 6 more red balls are put in the box and a ball is drawn at random, the probability of drawing a red ball doubles than what it was before. Find the number of red balls in the bag.Given,
Total balls in the box = 12
Red balls in the box = x
Probability of drawing a red ball = (๐๐ข๐๐๐๐ ๐๐ ๐๐๐ ๐๐๐๐๐ )/(๐๐๐ก๐๐ ๐๐ข๐๐๐๐ ๐๐ ๐๐๐๐๐ )
= ๐ฅ/12
If 6 more red balls are put in the box,
Total number of balls = 12 + 6 = 18
Number of red balls = x + 6
Probability of drawing a red balls = (๐ฅ + 6)/18
Given, after adding 6 balls, probability of drawing a red ball is
double of what it was before.
โด New Probability = 2 ร Old probability
(๐ฅ + 6)/18 = 2 ร ๐ฅ/12
(๐ฅ + 6)/18 = ๐ฅ/6
6 [(๐ฅ + 6)/18] = ๐ฅ
(๐ฅ + 6)/3 = ๐ฅ
๐ฅ+6 = 3๐ฅ
6 = 3๐ฅโ๐ฅ
6 = 2๐ฅ
2๐ฅ = 6
๐ฅ" = " 6/2
๐ฅ = 3
โด Number of red balls = x = 3

Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He has been teaching from the past 10 years. He provides courses for Maths and Science at Teachoo.