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Some people finished the work in 40 days. But it will be completed in 30 days when joined 10 people. Find the number of people who were at the starting.

Solution :

Let the no. of people at the beginning be $x$.

$\therefore$ From the question,

\begin{aligned} & 40 x=(x+10) \times 30 \\ \Rightarrow & 40 x=30 x+300 \\ \Rightarrow & 40 x-30 x=300 \\ \Rightarrow & 10 x=300 \\ \therefore & x=\frac{300}{10}=30 \text { people; Ans. } \end{aligned}

Trick : Required people

$$\begin{array}{l} =\frac{10 \times 30}{40-30} \\ =\frac{10 \times 30}{10}=30 ; \text { Ans. } \end{array}$$

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