我有d唔識分數除法,急!(10點)

1.
(15/28) / 21 <-----除21=1/21
=(15/28) x (1/21)
=(5/28) x (1/7) <-----=約簡
=5/196

2.
(3/4/15) / (5/1/4) <-------變假分數先
=(49/15) / (21/4)
=(49/15) x (4/21)
=(7/15) x (4/3) <-----=約簡
=28/45

28分之15除以21＝

15 21
--- 除 ---
28 1

15 1
--- 乘 ---
28 21

5 1
--- 乘 ---
28 7

5 x 1 5
--------- = --------
28 x 7 196


3又15分之4除以5又4分之1＝

4 1
3 ------- 除 5 -----
15 4

49 21
----- 除 ------
15 4

49 4
-----乘 ------
15 21

7 4
-----乘 ------
15 3

7 x 4
--------
15 x 3

28
----
45