x/30-x/20=-x/60原式=-60
X/(X/30+X/20)=X/[X(1/30+1/20)]=1/[(1/30+1/20)]=1/(2/60+3/60)=1/(1/12)=1*12=12
=X/(1/30*X+1/20*X)=X/(1/12*X)=X/X/1/12=12