Let us first calculate the number of bags I must have purchased if I paid Rs. 6.50 for Blue and Green marbles.
One bag has 2 Blue marbles, 1 Green marble and 5 Black marbles.
Cost of 2 Blue marbles = 2 x 20 p = 40 p.
Cost of 1 Green marble = 90 p.
I spend Rs. 6.50 on Blue and Green marbles, therefore the number of bags I must have bought = $\large\frac{6.50}{1.30}$$ = 5$
One bag has 5 Black marbles, therefore I have purchased 5 x 5 = 25 Black marbles.
Therefore the price I have paid for Black marbles = 25 x 5 p = 125 p = Rs. 1.25