How much Darjeeling must the merchant add to his 40 kg blend to raise its cost to ₹420 a kg?

Explanation

1. 01

   Alligate the blend at ₹390 with Darjeeling at ₹480: blend : Darjeeling $= (480 - 420) : (420 - 390) = 60 : 30 = 2 : 1$.

2. 02

   For 40 kg of blend, 20 kg of Darjeeling. Check: $\frac{15{,}600 + 9{,}600}{60} = 420$.