A taxi-driver, filled his car petrol tank with $40$ litres of petrol on Monday. The next day, he filled the tank with $50$ litres of petrol. If the petrol costs $Rs.44$ per litre, how much did he spend in all on petrol?

Petrol filled on Monday $=40$ litres

Petrol filled on the next day (Tuesday) $=50$ litres

Cost of $1$ litre of petrol $=Rs.44$

Now, the amount spent in overall petrol $=Rs.(44\times (40+50\left)\right)$

$=44\times 90$
$=44\times (100-10)$

Since, Distributive property over subtraction:

$a\times (b-c)=(a\times b)-(a\times c)$

$\Rightarrow 44\times (100-10)$

$=44\times 100-44\times 10$

$=4400–440$

$=3960$

Therefore, he spent $Rs.3960$ on petrol.