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

Petrol filled on Monday = 40 liters
Petrol filled on next day = 50liters
Total petrol filled = 90liters
Now, Cost of 1 liter petrol = Rs. 44
Cost of 90 liters petrol = 44 x90
= 44 x (100 – 10)
= 44 x 100 – 44 x10
= 4400 – 440
= Rs. 3960
Therefore, he spent Rs. 3960 on petrol.