A simple way to solve this is to divide how much the balloon descended by how quickly it descended. So you take 99.9 m ÷ 12 s and solve for the dividend, which would be 8.325 m/s. If you need to round to the nearest hundredth, you would round the 2 to a 3 because the thousandth number is a 5, and you always round up when the neighboring number is 5 or higher (and round down when the neighboring number is 4 and lower). Cheers!
