Learn these halves => 
half of 20 = 10
half of 40 = 20
half of 60 = 30
half of 80 = 40

To work out half of 30, partition it into 20 and 10.
Half of 20 = 10, Half of 10 = 5, So half of 30 = 10 + 5 = 15.

Example 1 => To work out half of 50 =>
Partition into 40 and 10 =>
half of 40 = 20
half of 10 = 5
so half of 50 = 20 + 5 = 25

Example 2 =>
To work out half of 70 =>
Partition into 60 and 10 =>
half of 60 = 30 half of 10 = 5 so half of 70 = 30 + 5 = 35

Now try a few practice questions!