Question

What is the least common denominator when adding the fractions a / 2, b / 3, c / 9, and d / 15?

A. 45
B. 90
C. 135
D. 270
E. 810
Ms. Sue
Only three of those are divisible by 2.

Which one is also divisible by 3, 9, and 15?

Will it be 90?
Ms. Sue
Yes. 90 is correct.
Because 2 x 9 x 15 = 90
Ms. Sue
No.

Oh. I apologize.
2 x 9 x 5 = 90. I realized my mistake.
I'm sorry. I don't know what I'm doing.
Ms. Sue
Study this site carefully.

http://www.mathsisfun.com/least-common-denominator.html
2: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20
3: 3, 6, 9, 12, 15, 18, 21, 24, 27, 30
9: 9, 18, 27, 36, 45, 54, 63, 72, 81, 90
15: 15, 30, 45, 60, 75, 90, 105, 120, 135, 150

so the LCD is 90

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