Let b represent blue marbles
g represent green marbles
Then,
We know that there are three times as many blues as green ones, so
b=3g
Next,
the bag contains 280 marbles total, so:
b+g=280 marbles
Substitute b=3g into this equation, to get
3g+g=280 marbles
4g=280 marbles
Divided 4 to each side
4g/4=280/4
g=70 marbles
b=3r
b=3(70)
b=210 marbles
Check:
b=3r
210=3(70)
210=210
b+g=280
210+70=280
280=280. As a result, there area 210 blue marbles. Hope it help!