Signed Quantities & Zero

How the lesson starts: Signed Quantities & Zero

Pia and Otto explore
Meet Explorer Pia and her buddy Otto! Pia climbs a hill 60 m above sea level. Otto crawls into a cave 30 m below sea level. How do we show above and below with numbers? We use positive and negative numbers, and a zero that means something!
Sea level is zero
Put them on a number line. Otto's cave is 30 m below sea level, so it's −30. Sea level sits at 0, the starting point. Pia's hill is 60 m above, so it's positive 60. Negative means below zero, and positive means above it.
Words into numbers
Signed numbers are everywhere. A night 9° below zero is −9°. An elevator 4 floors below ground stops at floor −4. A $20 deposit adds money, so it's positive 20. A $20 withdrawal takes money out, so it's −20. And no change at all is 0.