帮忙翻译这句话

刺激1995UID: 2808508

And when you've written 31,250 letters, as my earlier calculations indicate I may have written in my practice so far, a fourth of them before I started writing letters on computer in 1980,you really begin to appreciate the ability to cut and paste text from prior letters.
这句话的意思是四分之一的信件是在1980年之前写的吗？还是说开始使用电脑后，有四分之一的信件时用电脑写的？

jiang08UID: 2391146

前者。
关键：a fourth of them的them，指代前文出现过的内容，即所有写过的信
a fourth of them before I started writing letters on computer in 1980 整个半句是31,250 letters的同位语

刺激1995UID: 2808508

多谢jiang版