How much tea at Rs. 4 a kg should be added to 15 kg of tea at Rs. 10 per kg so that the mixture be worth Rs. 6.50 a kg?
Let x kg of tea be added.
So, 4x+15×10=(x+15)×6.50
4x+150=13x2+15×132
8x+300=13x+195
13x−8x=300−195
5x=105
x=21
∴ 21 kg of tea should be added.