I charged him 5% simple interest on the principal per week. 

我每周向他收取本金的5％作为单利

So what is it simple interest ?

那么什么是单利呢？

Make sure it's a simple interest loan.

切记，这只是一种简单的贷款利息。

At the end of the first article we had a simple interest calculator.

在第一篇文章终了，我们有了一个简单的利率计算器。

There is an important distinction between compound interest and simple interest.

复利与单利之间有一个重要的区别。

Simple interest rate is the most simple person family, and most will be familiar.

单利是整个利率家族最单纯的人物，也是最为大家所熟知的。