What you need to know
Fractions can look different but be worth the same amount, and the same fraction can be written in many ways. The key idea behind everything in this guide is simple: whatever you do to the numerator, do to the denominator (as long as you multiply or divide). This lets you rename a fraction so that you can simplify it, compare it with another fraction, or add it to one.
Equivalent fractions. Multiply or divide the top and bottom by the same number and the value does not change: 35 = 610 = 915. All three are the same point on the number line.
Simplifying. A fraction is in its simplest form when the only number that divides into both the numerator and the denominator is 1. To simplify, divide both by their highest common factor (HCF). If you cannot spot the HCF, divide by any common factor and repeat.
Simplify 812 and 1520
- 812: factors of 8 are 1, 2, 4, 8 and factors of 12 are 1, 2, 3, 4, 6, 12, so the HCF is 4.
- 8 ÷ 4 = 2 and 12 ÷ 4 = 3, so 812 = [frac 2/3].
- 1520: both numbers end in 5 or 0, so 5 divides into both.
- 15 ÷ 5 = 3 and 20 ÷ 5 = 4, so 1520 = [frac 3/4]. Check: 3 and 4 share no factor except 1, so you are finished.
Comparing fractions. If the denominators are the same, the fraction with the bigger numerator is bigger: 58 > 38. If the denominators are different, rename both fractions with a common denominator first.
Which is larger: 23 or 34?
- Find a common denominator. 12 is a multiple of both 3 and 4.
- 23 = 812 (multiply top and bottom by 4).
- 34 = 912 (multiply top and bottom by 3).
- 9 twelfths is more than 8 twelfths, so [frac 3/4] is larger.
Quick check using benchmarks: compare each fraction with 12. 38 is less than a half (half of 8 is 4) and 59 is more than a half (half of 9 is 4.5), so 59 is larger without any renaming.
Adding and subtracting with the same denominator. When the denominators match, the pieces are the same size. Add or subtract the numerators only and keep the denominator. Simplify the answer if you can.
710 − 310, then 56 + 36
- 710 − 310: 7 − 3 = 4, so the answer is 410.
- Simplify: 4 and 10 share the factor 2, so 410 = [frac 2/5].
- 56 + 36: 5 + 3 = 8, so the answer is 86.
- This is more than a whole. 6 sixths make 1, leaving 2 sixths: 86 = 126.
- Simplify the fraction part: 126 = 1[frac 1/3].
Common mistake: adding the denominators as well. 38 + 28 is 58, not 516. The denominator just names the size of the piece; it does not get added.
Check yourself
- Simplify 69.
- 47 = ?21
- Which is larger: 58 or 34?
- 29 + 59 = ?
- 910 − 410 = ? (simplify your answer)
- Simplify 1216.
Answers: 1. 23 2. 12 3. 34 (it equals 68) 4. 79 5. 510 = 12 6. 34