Answer :
let the no. of green marbles be x
no of blue marbles = 2x
no of red marbles = 3*2x = 6x
total marbles = 765
so, x+2x+6x = 765
or, x = 765/9 = 85
no of red marbles = 6*85 = 510
no of blue marbles = 2x
no of red marbles = 3*2x = 6x
total marbles = 765
so, x+2x+6x = 765
or, x = 765/9 = 85
no of red marbles = 6*85 = 510