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 ₹ 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

Total petrol filled = (40+50)litres=90 litres

Now, cost of 1 litre of petrol = ₹ 44

Cost of 90 litres petrol = ₹ ($44\times 90$)

For simplifying our calculations, we write

90 = (100 - 10)

So,

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

Using the distributive property,

a×(b-c) = (a×b) - (a×c)

we take 44 as common

= $(44\times 100)–(44\times 10)$

= 4400 – 440

= ₹ 3960

Therefore, he spent ₹ 3960 on petrol.