Abox contains 210 marbles which are either red or blue. there are 12 more red marbles than blue marbles. how many red marbles are in the box?

Let x and y be the number of red and blue marbles, respectively.

Since there are 12 more red marbles than blue ones, we have an equation: 
x- y= 12 (1)

On the other hand, we also know that:
x+ y= 210 (2)

x- y= 12 (1)
x+ y= 210 (2)

Take (1)+ (2), we have:
(x- y)+ (x+ y)= 12+ 210
⇒ 2x= 222
⇒ x= 222/2
⇒ x= 111

There are 111 red marbles.

Hope this helps.