Question

a 600 ml of bottle cost 1.20 rs and 2 litre cost 4.20 're which is the better buy and how much per liter

Answer 1

600 ml --------1.20 Rs----------1
2000ml -------4.20 Rs----------2

1
600 ml = 1.20 Rs
⇒1 ml = 1.20/600
⇒1ml = 120/60000
⇒1ml = 0.002 Rs.

2
2000 ml = 4.20 Rs
⇒1ml = 4.20/2000
⇒1ml = 420 /200000
⇒1ml = 0.021 Rs

∴ 600ml for Rs. 1.20 is better to buy then 2 liters for 4.20 Rs.

Thank You......!!!!!!!!!
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Yours, Jahnavi. 

Answer 2

given that
600ml=1.20------------- (i)
2 l=4.20
=>2000 ml= 4.20--------(ii)
=>1ml=4.20/2000
on equating eq^n (i)&(ii)
we get
4.20/2000=1.20/600
=>4.2/20=1.2/6
=>.21=.2
=>1.05=1 ml