90
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LCD(20, 6) = 60
210
LCD(5, 18) = 90
90
the answer is 45. You compute it by reducing each number to its prime factors, and taking the unique set. 9 factors to 3x3, 15 to 3x5, so the unique set is one 3 from the 9, one 3 that they share and one 5 from the 15. 3x3x5=45. Of course, 135 is also a common denominator but isn't the least. you can always find a CD by multiplying the two. The LCD will make simplification after the addition or subtraction of fractions much easier.