(2×5)分之1+(5×8)分之1+(8×11)分之1+……(20×23)分之1
=3×[(2×5)分之1+(5×8)分之1+(8×11)分之1+……(20×23)分之1]÷3
=[(2×5)分之3+(5×8)分之3+(8×11)分之3+……(20×23)分之3]÷3
=[1/2-1/5+1/5-1/8+1/8-1/11+1/11-1/14+1/14-1/17+1/17-1/20+1/20-1/23]÷3
=(1/2-1/23)÷3
=(23-2)/46*3
=21/46*3
=7/46