Download Extra Practice = Bonus Points

Extra Practice = Bonus Points!
Find the GCF and LCM of the following:
a) 60 and 72
b) 32 and 64
c) 16 and 20
d) 105 and 180
e) 18 and 45
f) 10 and 12
g) 14 and 21
h) 77 and 98
i) 9 and 27
*Remember, you can use any of the three methods shown in class…
1. GCF: Write down all factors of both numbers and circle the largest one they
have in common. LCM: Write down multiples of the numbers, and search for the
lowest they have in common. Examples:
Factors of 12 are 1, 2, 3, 4, 6, 12
Multiples of 12 are 12, 24, 36, 48, 60, …
8 are 1, 2, 4, 8
8 are 8, 16, 24, 32, 40, …
2. Use factor trees to factor numbers into a product of its prime numbers.
60
72
3 x 20
4 x 5
2 x 2
3 x 24
4
x
6
2 x 2 2 x 3
Then… Line up numbers to see both GCF and LCM. Example:
60 = 2 x 2 x 3 x 5
72 = 2 x 2 x 3 x _ x 2 x 3
GCF uses the columns that have two numbers in it: (2 x 2 x 3) = 12
LCM uses any column that has a number in it: (2 x 2 x 3 x 5 x 2 x 3) = 360
3. Using the repeated division method shown in class today. Remember you may
only use a prime number to divide into the numbers. Example:
2 ) 60 72
3 ) 30 36
2 ) 10 12
2) 5
6
5
3
4. Go and have a drink of
*Once you can’t divide by a prime number, you’ve
found the GCF (2 x 3 x 2) = 12
Now you also have your LCM: all the prime
numbers used along the side and bottom
(2 x 3 x 2 x 2 x 5 x 3) = 360
(This should look like an ‘L’ or hockey stick!
and 2
You’ve earned it! Whew!