# A box contains 11 marbles, 7 red, and 4 green.

A box contains 11 marbles, 7 red, and 4 green. Five of these marbles are removed at random. If the probability of drawing a green marble is now 0.5, how many red marbles were removed?
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Finn Lucero
Given:
Red marbles $=7$
Green marbles $=4$
Total no of marbles $=11$
Number of Marbles removed $=5$
Marbles in the box $=11-5=6$
Probability of drawing one green marble is $=0.5$
Among the remaining 6 marbles, x number of marbles are green.
Then Probability of drawing one green marble is = number of green marbles / Total Number of marbles.

We know
Then
$\frac{x}{6}=0.5$
$x=0.5×6=3$
After 5 marbles are removed from the box, the number of green marbles remaining is 3.
It means out of 4 green marbles, one green marble has been removed.
Out of 5 marbles one marble is green. Then the other four must be red.
Then number of red marbles removed were $=4$.