Grade 5 Mathematics
You can only add or subtract fractions when they have the SAME DENOMINATOR!
The denominator tells us the SIZE of each piece. You can't add halves and thirds directly - that's like adding apples and oranges!
Solution: Find a common denominator first, then add or subtract the numerators.
The LCD is the smallest number that BOTH denominators divide into evenly.
Method: List the multiples of each denominator until you find a match!
Example: Find LCD of 3 and 4
Multiples of 3: 3, 6, 9, 12, 15...
Multiples of 4: 4, 8, 12, 16...
LCD = 12
Find the LCD of 2 and 3. Multiples of 2: 2, 4, 6... Multiples of 3: 3, 6... LCD = 6
Convert each fraction to an equivalent fraction with denominator 6:
12 = 1 x 32 x 3 = 36
13 = 1 x 23 x 2 = 26
Add the numerators, keep the denominator:
Find the LCD of 4 and 3. LCD = 12
Convert: 3/4 = 9/12 (multiply by 3) and 1/3 = 4/12 (multiply by 4)
Subtract: 9/12 - 4/12 = 5/12
WRONG: 1/2 + 1/3 = 2/5 (Adding 1+1 and 2+3 is INCORRECT!)
RIGHT: Find common denominator first! 1/2 + 1/3 = 3/6 + 2/6 = 5/6
1. 14 + 12 =
LCD = _____ Convert: 1/4 = ___/___ 1/2 = ___/___
2. 23 + 16 =
LCD = _____ Convert: 2/3 = ___/___ 1/6 = ___/___
3. 56 - 12 =
LCD = _____ Convert: 5/6 = ___/___ 1/2 = ___/___
4. 34 - 25 =
LCD = _____ Show your work: