Green marbles = x.
There are 4 times as many red marbles as green marbles, so red marbles = 4x.
There are half as many blue marbles as green marbles, so blue marbles = 0.5x
There are 33 marbles in total, so x+4x+0.5x=33.
[tex]x+4x+0.5x=33 \\ 5.5x=33 \\ x=\frac{33}{5.5}=\frac{330}{55}=\frac{30 \times 11}{5 \times 11}=\frac{30}{5}=6 \\ \\
0.5x=0.5 \times 6=3[/tex]
There are 3 blue marbles.