Explanation:

Ratio of tea and water in 729 ml is 7:2
->tea in 729 ml of combination = [(7/9)*(729)] ml = 567 ml
water in 729 ml of combination = 729 – 567 = 162
->If “a” be the some of water included to new combination, with the ratio 7:3
some of water in the new combination= (162 + a )
Then (7/3) = [ (567) / (162+a) ]
=> 7 (162 + a) = 3 x 567
=> 1134 + 7a = 1701
7a = 1701 – 1134
A = 567/7 = 81 ml
Some of water included to new combination = 81 ml.