20/19 x 21/1
=20/19 x (19+2)
=20+40/19
=22+2/19
或者
20/19 x 21/1
=(1+1/19) x 21/1
=(1+1/19)x 21
=21+1/19x(19+2)
=21+1+2/19
=22+2/19