Let us first calculate the cost of Blue and Green marbles present in a bag of 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.
Therefore the cost of 2 Blue and 1 Green marble = 40 p + 90 p = Rs. 1.30
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$