此题多解：
①
1/20+1/30+1/60
=3/60+2/60+1/60
=6/60
=1/10，
a=20，b=30，c=60，
②
1/15+1/45+1/90
=6/90+2/90+1/90
=9/90
=1/10
a=15，b=45，c=90.
