I don't know if smartphones cause brain cancer or not, but they definitely cause head trauma. In fact, the general population's inability to do simple head math is one of the greatest collateral damages of the omnipresent calculator era. Head math is still useful. I mean, what if you get stranded on a deserted island?

But srsly. Head math lets conversations involving simple arithmetic decisions continue fluidly. It allows you to stay hands-free while looking through the sales rack. And candidly, it just impresses people—in a non-arrogant way, which is the best way, right?

Here are a few you should know.

To multiply by 4, double the number twice.

Example: 17 x 4. Think 34, then 68.

To multiply by 5, halve the number, then multiply by 10.

Example: 72 x 5. Think 36, then 360.

To calculate a 15% tip, halve the number, then add it to the number, then move the decimal one spot to the left.

Example: 23 x 15%. Think 11.5, then 23 + 11.5 = 34.5, then 3.45.

To calculate a 20% tip, double the number, then move the decimal one spot left.

Example: 41.25 x 20%. Think 82.50, then 8.25.

Note: this is just another reason to tip well. The math is easier.

Instead of subtracting, add “up” to the higher number. It’s way easier mentally.

Example: 365 - 280. Think 20 gets me from 280 to 300, then 65 gets me to 365. Add the pieces: 20 + 65 = 85.