Answer:
11 marbles.
Step-by-step explanation:
Given: Total number of marbles= 21.
The number of blue marbles is 9 less than 2 times the number of red marbles.
Now, finding the number of blue marbles.
Lets assume the number of red marbles be "x".
∴ number of blue marbles= [tex](2\times x-9)[/tex]= (2x-9)
As given total number of marbles is 21.
we can write, [tex]x+ (2x-9)= 21[/tex]
Opening parenthesis.
⇒ [tex]3x-9= 21\ marbles[/tex]
Adding 9 on both side.
⇒ [tex]3x= 30[/tex]
cross multiplying both side
∴ x= [tex]\frac{30}{3} = 10\ marbles[/tex]
Red marbles= 10 marbles.
Now, subtituting the number of red marbles to find number of blue marbles.
Blue marbles= (2x-9)
Number of blue marbles= [tex]( 2\times 10-9)= 20-9[/tex]
∴ Number of blue marbles= 11 marbles.
