Some common denominators are not very convenient due to their large size. It's a good idea to find the lowest common denominator (LCD). The first check to make with all pairs of fractions is whether the smallest denominator is a factor of the other one. e.g. 1/4 and 3/20 ... 4 is a factor of 20! So, whilst we could multiply the denominators (and correct the numerators) to give 20/80 and 12/80 (which we could simplify), it's much easier to spot the factor (4) and alter only this fraction: 1/4 and 3/20 becomes 5/20 and 3/20.
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