1/4+1/2
=1/4+2/4
=3/4
2/5+3/4
=8/20+15/20
=23/20
3/4-2/5
=15/20-8/20
=7/20
1/2+2/5
=5/10+4/10
=9/10
2/7+2/5
=10/35+14/35
=24/35
3/10+2/5
=3/10+4/10
=7/10
5/8-1/2
=5/8-4/8
=1/8
3/4-1/2
=3/4-2/4
=1/4
9/10-1/2
=9/10-5/10
=4/10
=2/5
①乙桶裝油=1/3-1/6
=2/6-1/6
=1/6 千克
甲乙兩桶共裝油=1/3+1/6
=2/6+1/6
=3/6
=1/2 千克
②
(1)：
2/5+3/8
=16/40+15/40
=31/40
小麥和大豆的種植面積占總面積的31/40
(2)：
1-31/40
=40/40-31/40
=9/40
玉米占總面積的9/40