A tea of 10 Rs kg total amount is 15 kg How much amount is mixed of 4 Rs kg so that rate of mixture ...........

Explanation:

10 Rs/kg total amount of 15 kg
x amount of 4 Rs/kg 
resultant mixture rate 6.50 Rs/kg
using mixture method 
10-6.50:6.50-4
3.5 :2.5
7:5
ratio of amount is 7:5
7/5 =x/15
x=21 kg