I don't want to make them TOO easy for you! I keep a calculator in my baking gadgets drawer and use it frequently, as well as the calculator app on my iPhone.
I got it. I didn't want to run back to the apt. for the calculator (computer is in the still being remodeled house), so I worked it out with pen and paper.
When I was teaching math, this was part of my classic response to "Why do I have to know algebra?" Our discussion began with "Have you ever had to clean spilled-over cake batter from the bottom of your oven?" and ended with "Well, you can either do the math or maybe clean your oven." For sure, I'd hear the same "Why do I have to know algebra?" again soon, but eventually some vivid real-life examples made an impact.
Some years ago my wife was working for Upward Bound and taught a course on 'math survival skills'. She had a number of kitchen math examples. Most of them were pretty basic, like how many loaves of bread do you need to make 16 sandwiches if each loaf has 12 slices of bread?