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Example: NCERT, EX -12.2, Pg- 256
4(d) 200 mL : 2.5 litre and Rs 4 : Rs 50
200 mL : 2.5 litre = 200 mL : 2500 mL ( 1L =1000 mL, 2.5 L = 2.5 x 1000 mL =
2500mL)
Here let a = 200 , b = 2500
c = 4 , d = 50
a x d = 200 x 50 = 10,000
c x b = 4 x 2500 = 10,000
As, a x d = c x b
Therefore, 200 mL : 2.5 litre and Rs 4 : Rs 50 are in proportion

Q. Given that a : 15 and 2: 5 are in proportion. Find the value of ‘a’?
Solution: Since a : 15 and 2: 5 are in proportion
We have a x 5 = 15 x 2
or, a x 5 = 30

or, a =

or, a = 6
Therefore, a = 6
Video Link: https://www.khanacademy.org/math/pre-algebra/pre-algebra-ratios-rates/pre-
algebra-write-and-solve-proportions/v/find-an-unknown-in-a-proportion