Because the marble is replaced after being drawn, and so the number of marbles of a given color remains the same, the event of drawing any given color is independent of what was drawn before. So
X=12.5 If the perimeter is 90 90=3x+3x+15 Combine like terms 90=6x+15 Subtracts 15 from both sides 75=6x Divide by six to get x by itself 12.5=x Does this help?