Page 164 - Start Up Mathematics_8 (Non CCE)

1
Mamta can alone do of the work in 1 hour.
72
1 72
\ Mamta can alone do 1 work in 1 ÷ hours = 1 × hours = 72 hours
72 1
So, Mamta can alone finish the work in 72 hours.

Example 4: Ram and Shyam can together paint a wall in 25 minutes. Ram can alone paint the same wall
in 50 minutes. They work together for 20 minutes, then Ram goes away. In how many minutes
will Shyam paint the remaining part of the wall?

Solution: Ram and Shyam can together paint a wall in 25 minutes.
1
\ Ram’s and Shyam’s 1 minute work =
25
Ram can alone paint the wall in 50 minutes.
1
\ Ram’s 1 minute work =
50
\ Shyam’s 1 minute work = (Ram’s and Shyam’s 1 minute work)
– (Ram’s 1 minute work)
1 1 21- 1
= - = =
25 50 50 50
1 4
The amount of work that can be done by Ram and Shyam in 20 minutes = 20 × =
25 5
4 1
Remaining work = 1 – =
5 5
1
Shyam can complete of the work in 1 minute.
50
1 50
\ Shyam can complete 1 work in 1 ÷ minute = 1 × minutes = 50 minutes
50 1
1 1
Hence, of remaining work will be completed by Shyam in 50 × = 10 minutes
5 5
\ Shyam will paint the remaining part of the wall in 10 minutes.
Example 5: Spinning 4 hours daily, Santosh can spin 3 kg of cotton balls in 15 days. If she spins 5 hours
daily, how many days will she take to spin 8 kg of cotton balls?
Solution: Spinning 4 hours daily, Santosh can spin 3 kg of cotton balls in 15 days.

15
\ Spinning 4 hours daily, Santosh can spin 1 kg of cotton balls in = 5 days
3
fi Spinning 1 hour daily, Santosh can spin 1 kg of cotton balls in 5 × 4 = 20 days
fi Spinning 1 hour daily, Santosh can spin 8 kg of cotton balls in 20 × 8 = 160 days

160
fi Spinning 5 hours daily, Santosh can spin 8 kg of cotton balls in = 32 days
5
Hence, if Santosh spins 5 hours daily, she will take 32 days to spin 8 kg of cotton balls.
Example 6: 10 workers can earn ` 4,800 in 5 days. What amount will 8 workers earn in 9 days?

Solution: In 5 days:
Since the number of days are fixed, lesser the number of workers, less is the amount earned.
Thus, the number of workers are in the direct variation with the amount earned.

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