What negative numbers mean

1Positive and negative numbers

Positive numbers are greater than 0: 1, 2, 3 … We can write them with a plus sign, +3, or without it, 3.

Negative numbers are less than 0. We always write them with a minus sign: −1, −2, −3 … We read −3 as "negative three" or "minus three".

Zero is neither positive nor negative. The whole numbers, their negatives and zero together are called integers.

2What does zero mean?

The meaning of a negative number depends on what zero means in the story.

• Temperature: 0 °C is the temperature at which water freezes. −3 °C is 3 degrees colder than that.
• Height: if 0 is the surface of Lake Kivu, a fish swimming 12 m under the water is at −12 m, and a house 40 m up the hill is at +40 m.
• Floors: if 0 is the ground floor of a building in Kigali, the car park two floors under the ground is floor −2.
• Money: if Mutesi has 0 Frw and owes the shopkeeper 500 Frw, her money is −500 Frw.

The Musanze thermometer shows 6 °C: six degrees above zero. The Karisimbi thermometer shows −3 °C: three degrees below zero.

Try it yourself

Work each one in your exercise book first. Then tap Show answer to check.