Subtracting integers

Subtraction has a meaning you may know already: making two amounts equal. I have ₹10 and my sister has ₹6. How much more does she need to have as much as me? We can write 6 + ? = 10, or 10 - 6 = ?. Subtraction finds the missing number to be added.

Now use this in Bela's Building of Fun. Your starting floor is the Art Centre (+2) and your target floor is the Sports Centre (+5). You need to go three floors up, so you press +3. As a subtraction:

• Target Floor - Starting Floor = Movement needed, so (+5) - (+2) = +3.

The movement can be up or down.

• Target -1, starting -2: go one floor up, so (-1) - (-2) = +1.
• Target -1, starting +3: go four floors down, so (-1) - (+3) = -4.
• Target +2, starting -2: two floors up to the ground floor and two more, so (+2) - (-2) = +4.

Always read it as 'where I want to be' minus 'where I am'.

The book also has a mine, with levels marked in metres above the ground (positive) and below it (negative). The lift goes from level -50 to level +40: 50 metres up to ground level and 40 more, so (+40) - (-50) = +90. From level +40 down to level -90: 40 metres down to the ground and 90 more, so (-90) - (+40) = -130.