a) 28=2²x7
21=3x7
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[28,21]=2²x3x7=84
b) 30=2x3x5
45=3²x5
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[30,45]=2x3²x5=90
c) 25=5²
60=2²x3x5
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[25,60]=2²x3x5²=4x3x25=300
d)
15=3x5
35=5x7
42=2x3x7
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[15,35,42]=2x3x5x7=210
e)
25=5²
40=2³x5
45=3²x5
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[25,40,45]=2³x3²x5²=8x9x25=1800