Let G, B and Y denotes the number of green, blue and yellow marbles.
Given that:
[tex]\begin{gathered} G+B+Y=694 \\ B=3Y \\ B=G+90 \end{gathered}[/tex]
From this,
[tex]Y=\frac{B}{3},G=B-90[/tex]
Substitute the vlaues of Y and G into the equation G+B+Y=694.
[tex]\begin{gathered} B-90+B+\frac{B}{3}=694 \\ \frac{7B}{3}-90=694 \\ \frac{7B}{3}=784 \\ B=\frac{784\cdot3}{7} \\ =336 \end{gathered}[/tex]
Keith had 336 blue marbles.
