First fractions · Usually met around P3
Adding and taking away like fractions
Keep the bottom, add or take away the tops.
How to do it
Method
Keep the bottom, add or take away the tops
27 + 37 = 57
Another way
Count parts on a strip
2 parts and 3 more make 5 → 57
In full
With the same denominator the parts are the same size, so just count parts: keep the bottom, add or take away the tops. 27 + 37 = 57; 69 − 49 = 29. With three fractions go left to right: 19 + 49 + 29 = 59 + 29 = 79. When the top and bottom are equal it is one whole, 1: 35 + 25 = 55 = 1. For 1 − 38, think of 1 as 88: 88 − 38 = 58. Never add the denominators. Or draw a strip and count the parts.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
69 + 19 = □
Answer in simplest form
Show the answer and working
Answer: 79
Keep the denominator and add the numerators: 6 + 1 = 7, so 79.
Example 2
1 − 59 = □
Answer in simplest form
Show the answer and working
Answer: 49
Think of 1 as 99: 99 − 59 = 49.
Example 3
412 + 412 − 312 = □
Answer in simplest form
Show the answer and working
Answer: 512
Left to right: 4 + 4 = 8, 8 − 3 = 5, so 512.
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