A car owner buys petrol at Rs.7.50, Rs. 8 and Rs. 8.50 per litre for three successive years. What approximately is the average cost per litre of petrol if he spends Rs. 4000 each year?

A. Rs. 7.98 B. Rs. 8
C. Rs. 8.50 D. Rs. 9

• petrol used in 1st yr = 4000/7.5 = 533.33 ltr
  petrol used in 2nd yr = 4000/8 = 500 ltr
  petrol used in 3 rd year = 4000/8.5= 470.59 ltr
  
  Total petrol used for Rs.12000(4000/- for each yaer) in 3rd yrs = 1503.92 ltr
  
  Average cost per litre = 12000/1503.92 = Rs 7.98
• 10 years agoHelpfull: Yes(7) No(1)
• Total Cost = 4000 × 3
  
  Total diesel used = 40007.5+40008+40008.5
  
  Average cost per litre of diesel = 4000×3(40007.5+40008+40008.5)=3(17.5+18+18.5)
  
  It is important how you proceed from this stage. Remember time is very important and if we solve this in the normal method, it may take lot of time. Instead, we can find out the approximate value easily and select the right answer from the given choices.
  
  In this case, answer =3(17.5+18+18.5)≈3(18+18+18)≈3(38)≈8
  
  We got that answer as approximately equal to 8. From the given choices, the answer can be 8 or 7.98 or 8.1 . But which one from these?
  
  It is easy to figure out. We approximated the denominator,(17.5+18+18.5) to 38. However
  17.5+18.5=18−0.5+18+0.5=8+0.5+8−0.5(8−0.5)(8+0.5)=16(82−0.52)[because (a−b)(a+b)=a2−b2]=1664−0.25
  
  i.e., 17.5+18.5=16(64−0.25)
  We know that 18+18=14=1664
  
  => 17.5+18.5>18+18
  
  Early we had approximated the denominator to 38. However, from the above mentioned equations, now we know that actual denominator is slightly greater than 38. It means answer is slightly lesser than 8. Hence we can pick the choice 7.98 as the answer
  
  Try to remember such relations between numbers which can save lot of time in calculations.
  
• 7 years agoHelpfull: Yes(0) No(0)