Checking your answers when doing algebra is always a good idea, just like reconciling your checkbook with your bank statement is a wise move. Check your answers on two levels:

Level 1: Does the answer make any sense?

If your checkbook shows a balance of $40 million, does that tidy sum seem accurate — or even possible? Sure, we'd all like to see that balance, but for most of us this inflated figure would be a red flag that something is wrong with our computations.

Level 2: Does putting the answer back into the problem give you a true statement? Does it work?

This is the more critical check because it gives you more exact information about your answer. The first level helps weed out the obvious errors. This is the final check.