Before defining rate, first we consider some examples.
(Ans). One dozen eggs cost = 24 Rs
Cost of 1 egg = 24/12=2Rs/egg
(Ans). In 60 liters distance travelled= 540km
In 1 liter distance travelled = 540/60=9km/liter
(Ans) In 5 hours boy gets = 120 Rs.
In 1 hour boy gets = 120/5=24Rs/hour
In each of the above examples, the two quantities that is the quantity in numerator and the quantity in denominator have different units, whereas in ratio both quantities have same units.
“The division of two quantities having different unit, is called RATE”
In the above examples, we have

Rate = 24/12 = 2Rs/egg

Rate = 540/60 = 9km/liter

Rate = 120/5 = 24Rs/hour
Example:
A car uses 40 liters of petrol to travel 320 km. How far can is travel for 32 liters?
Solution:
In 40 liters car traveled = 320km
In 1 liter car traveled = 320/40 = 6km/liter
In 32 liter car traveled = (8km/liter)(32liter) = 256km