I know that the circumference of a circle is given by the formula $C = 2\pi r$, where $r$ is the radius. So, to find the ratio of the two circumferences, I need to plug in the values of $r$ for each circle and simplify. For the first circle, $r = 1.125$, so $C_1 = 2\pi (1.125)$. For the second circle, I notice that the diameter is twice the radius, so $r = 2.25/2 = 1.125$ as well. That means $C_2 = 2\pi (1.125)$ too. So, the ratio of the circumferences is $C_1:C_2 = 2\pi (1.125):2\pi (1.125)$. This simplifies to $1:1$, since the factors of $2\pi$ and $1.125$ cancel out. So, the answer is $1:1$. 
Find the ratio of the circumference of a circle with radius 1.125 to the circumference of a circle with diameter 2.25. Express your answer in the form $a:b$ where $a$ and $b$ are relatively prime positive integers.