急:提问几道GRE数学题

急:提问几道GRE数学题
1.A/x + A/y=1.In this formula,x and y
represent the time needed for each of two workers to complete the job alone; A
represents the time it takes for both x and y to complete the job working
together.So each fraction represents the portion of the job completed by a
worker.
请解释该公式含义,没太看懂,尤其没明白A/x是什么意思
2.牛吃草问题：牧场上有一片匀速生长的草地,可供27头牛吃6周,或供23头牛吃9周,那么它可以供21头牛吃几周?（P86）
3.鸽巢原理：有0-9这十个数字分别写在10张卡片上,随机从这10张纸片中抓,问至少抓几个纸片才能保证所抓纸片上必有两个纸片上所写数字相加等于10?（P88）
彻底晕菜,
4.Fred is twice as old as Geri was 4 years
ago.Holly is five years older than Geri will be one year from now.
答案是F=2(G-4),H=G+6.为何不是F-4=2(G-4),H+1=G+6?