Fractions with unlike denominators · Usually met around P5
Adding and taking away mixed numbers
Fraction too small? Borrow 1 from the wholes.
How to do it
Method
Fraction too small? Borrow 1 from the wholes
4 16 − 1 34 = 3 1412 − 1 912 = 2 512
Another way
Improper fractions
256 − 74 = 5012 − 2112 = 2912
In full
Way 1: work the whole parts and the fractions separately. Give the fractions a common denominator; if they add up to more than 1, carry 1 to the wholes; if the fraction can't be taken away, borrow 1 from the wholes as a fraction: 4 16 − 1 34 = 3 1412 − 1 912 = 2 512. Way 2: change everything to improper fractions, work it out, and change back: 256 − 74 = 5012 − 2112 = 2912 = 2 512. Both give the same answer; way 1 is quicker with small numbers, way 2 is steadier when there is borrowing.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
4 59 + 3 12 = □ □/□
Fill in each box
Show the answer and working
Answer: 8，1，18
Add the wholes, 4 + 3; add the fractions over a common denominator and carry any whole: 8 118.
Example 2
4 15 − 1 34 = □ □/□
Fill in each box
Show the answer and working
Answer: 2，9，20
15 is less than 34, so borrow 1 from 4 as a fraction, then take away: 2 920.
Example 3
5 29 − 2 14 = 3 136
True or false?
Show the answer and working
Answer: False
29 is less than 14: borrow 1 from the wholes first; never swap it round to 14 − 29.
Practise it in the game →Fractions with unlike denominators · Free to start, unlimited questions, no download