Let the proportion of tea costing 6.92 used is x

So, the proportion of tea costing 7.77 will be (1-x)

Which gives us our LHS as 6.92x + 7.77 (1 -x)

Tea seller earns a profit of 17.5 percent on sales.

So the cost of sales = 8.8*(1-17.5%) = 

                            = 8.8*(1-17.5/100) 

                            = 8.8*(1-175/1000) 

                            = 8.8*{1000-175)/1000} 

                            = 8.8*{825/1000} 

                            = 8.8*0.825 

                            = 7.26

which forms our RHS

Now, Our equation is:

     6.92x + 7.77(1-x) = 7.26

=> 6.92x + 7.77 - 7.77x = 7.26

=> 6.92x  - 7.77x = 7.26 - 7.77

=> 0.85x = 0.51

=> x = 0.51/0.85

=> x = 51/85

=> x = 0.6 

Now, 1 - x = 1 - 0.6 = 0.4

So, the required ratio = 0.6/0.4 = 6/4 = 3/2 = 3 : 2