Answer:
49
Step-by-step explanation:
The probability of choosing a red marble is equal to the number of red marbles over the number of total marbles there are.
Therefore, let the number of red marbles be [tex]x[/tex].
We have the following equation:
[tex]\frac{x}{84}=\frac{7}{12}[/tex]
Cross-multiplying, we get:
[tex]12x=7\cdot 84,\\x=\frac{84\cdot 7}{12},\\x=\boxed{49}[/tex]
Therefore, there are 49 red marbles in the bag.