Answer:
You have 24 blue marbles.
Explanation:
We can solve this question by a system of equations.
I am going to say that:
x is the number of red marbles.
y is the number of blue marbles.
28 red and blue marbles
This means that

The ratio of red to blue is 1:6.
This means that for each red marble(x), we have 6 blue marbles(y). So
y = 6x.
Then




We want the number of blue marbles(y)



You have 24 blue marbles.