Make a proportion for blue which is 351/1200=x/20. Cross multiply and solve for x. Your answer for blue marble is 5.85, which is rounded to 6 blue marbles.
Make a proportion for green which is 482/1200=x/20. Cross multiply and solve for x. Your answer for green marble is 8.0333..., which is rounded to 8 green marbles.
Make a proportion for yellow which is 367/1200=x/20. Cross multiply and solve for x. Your answer for yellow marble is 6.116666.., which is about 6 yellow marbles.