The ratio of dry rice to water is 1 to 2. This means that water is two times more than dry rice. Since the chef uses 9.5 cups of dry rice, the chef needs 19 cups of water.
You can check by using fractions. 9.5 / 19 in simplified form is 1/2, which is 1:2 in ratio.
It would be 9.5:19 just multiply 9.5 by 2