Solving percents. I use this to predict gross and net of my paychecks when I do my budget.

Net               X
------    =    ------
Gross            100


(GrossX) = (Net × 100)

GrossX ÷ (Net × 100) = X

Example;

200             X
------    =   -------
250            100

Step A
200 × 100 = 20000

Now 250X = 20000

Step B
20000÷250=80

Therefore X=80
Also expressed as 80% of 250 = 200.

It's also kinda handy when figuring out sales and stuff at the store, sometimes.