He must work for 9 hours to earn Rs720.
What is Concept of direct proportion?
Whenever, two quantities are directly proportional to each other, they depend on each other directly that is if one increases then other also increases and if one decreases the other also decreases. It is denoted by the sign ∝.
In this question, the pay is directly proportional to the number of hours he worked.
Pay ∝ No. of hours worked
For 1 hour, he got Rs80 , then for x hours, he will get Rs720.
Putting it in mathematical form:
[tex]\dfrac{1}{x} =\dfrac{80}{720} \\\\\dfrac{1}{x} =\dfrac{1}{9} \\x=9[/tex]
Therefore, he worked for 9 hours to get required pay.
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