Explanation:
Here is how you do this problem:
we start off with the ratio of Darjeeling in the tea as 20% which is 1/5
every time we add (x) amount of Darjeeling, the total weight of the mixture would also increase by (x) amount
this idea can be represented by the expression
1 + x
---------- = 0.7
5 + x
I set this equal to 0.7 because our goal is to get it to 70% = 0.7
solving for x should get you to your answer