Subtraction Up to 20

Subtraction is a mathematical operation used to find the difference between two quantities, such as working out how many items have been taken away from a group. When a known starting amount decreases, subtracting the items that remain reveals exactly how many are missing.

How subtraction works

Subtraction takes a smaller number away from a larger number to determine what is left or what has been removed. To find an unknown missing quantity, one records the original total count and then counts the items that are currently present. Subtracting the current count from the starting count gives the precise difference between the two amounts.

Calculating the missing flowers

• On the blue plant, four flowers usually bloom every day, but only three were left, so four minus three showed that one flower was missing.
• On the yellow plant, six flowers usually bloom, but only four were left, so six minus four showed that two flowers were missing.
• On the red plant, nine flowers bloomed normally, but only five were left, so nine minus five showed that four flowers were missing.

Example from the garden

Jansi grew three different flower plants at home: a red plant, a yellow plant, and a blue plant. When she saw that blooms were gone one morning, she wrote down the normal blooming numbers in her notebook. By subtracting the flowers that were still on the plants from the usual totals, she calculated the exact number of flowers that had been taken from each plant.

Solving the mystery

To discover who took the missing flowers, Jansi woke up very early in the morning and hid herself to watch the garden. Her friend Subha arrived secretly and admitted that she had stolen the flowers without Jansi knowing. After Subha promised not to steal again, Jansi explained that she would have happily shared the flowers if asked, and the two friends bought another flower plant to grow together.