If \(3a = 4b = 5c\), then \(a:b:c = 20:15:12\).
Let \(3a = 4b = 5c = k\).
\(\therefore a = \cfrac{k}{3}, b = \cfrac{k}{4}, c = \cfrac{k}{5}\)
\(\therefore a:b:c = \cfrac{k}{3} : \cfrac{k}{4} : \cfrac{k}{5}\)
\(= \cfrac{1}{3} : \cfrac{1}{4} : \cfrac{1}{5}\)
\(= \cfrac{60}{3} : \cfrac{60}{4} : \cfrac{60}{5}\)
\(= 20:15:12\)