Work is force times distance: handling your calculator is just as much work as is figuring out what to write down. You will have more time to figure out what to write down if you wait until you've worked out solutions to all the problem you know how to work, before you use the calculator to compute numerical values. Picking up the calculator, putting it down, making the context shift in your mind,.., all take time away from the work that's earning you points on that test. So just do it once. I will say it again: deal with the numbers only once!