Question

100 lamps can be lighted 4 hrs per day for 10 days for Rs 200. How many lamps can be lighted 4 hrs per day for 10 days for Rs 600. [4 MARKS]

Answer 1

Concept: 1 Mark
Application: 3 Marks

$\begin{array}{cccc}Lamps& Days& Cost& Hours/Day\\ 100& 10& Rs.200& 4\\ ?& 10& Rs.600& 4\end{array}$

Clearly the days column and hours per day column remain constant

So, the table simplifies to

$\begin{array}{cc}Lamps& Cost\\ 100& Rs200\\ x& Rs600\end{array}$

This is a case of direct variation

So, cross multiplying we have

$100\times 600=200\times x$

$\Rightarrow \frac{100\times 600}{200}=x$

$\Rightarrow \frac{600}{2}=x$

$\Rightarrow 300=x$

$300Lamps$