There are 765 marbles in a box. There are twice as many blue marbles as green marbles. There are three times as many red marbles as blue. How many blue marbles are there in the box?

B=blue

g=green

r=red

2 times as many blue as green

b=2g

3 times as many red as blue

r=3b

subsitute 2g for b

r=3 (2g) = 6g

r=6g

r+g+b=765

so we subsitute

r=6g

g=g

b=2g

6g+g+2g=765

add like terms

9g=765

divide both sides by 9

g=85

subsitute

b=2b

b=2 (85)

b=170

170 blue