Go Forth and Multiply

Today I had some lab work to do after my clinic appointment. Luckily we get most Thursday afternoons off to do such lab work and other errands. I was really excited about the idea of getting my work done and getting home before 5:30.

I had to mix up some casts for one of my patients. Our dental stone comes in separate powder packets, and each one is clearly marked with how much water should be added to mix the proper consistency. The stone I was using called for 13 milliliters of water per bag of stone. I wanted to mix 3 bags of stone at once. So here's your math problem, straight out of the 3rd grade practice book:

Q: Lauren is mixing dental stone in the lab. She wants to mix 3 bags of stone, and each requires 13 ml of water. How many ml of water should she add to the 3 bags of stone?

A: 39 ml

So would someone please tell me why I used 69 ml of water?!?! 69 is not even close to 39. I took AP precalculus and calculus in high school, then took precal at the college level, and yet somehow simple mental math still sometimes escapes me.

Anyway, I tried to add more powder and salvage the situation, but basically I just ended up with soup instead of the nice, thick consistent mixture we aim for in our stone. The cost of adding too much water is that the stone takes much longer to set. Usually stone takes about 30 minutes to set. Because I'm math-tarded, mine will probably be dry sometime next Friday.

This only goes to show that my friend Trang is right about the lab rule. The rule is this:

Amount of time required to do a lab project = 2 x (Amount of time you think it will take) + 40 mins

I thought that making my cast would take 30 mins, but applying Trang's rule I see that really it would take (oh no, more math!) 100 minutes.

(But somebody better double check that math for me.)