Greetings,

I've just started taking a computer systems class, and although it wasn't a pre-requisite, my teacher is recommending that students take a programming course, specifically C++, prior to taking the computer systems class. The teacher said if we didn't, we would really struggle. I'm trying it out and I've encountered my first problem. I understand most of the solution given except for two aspects. If someone could please help me to understand those two aspects, it would help me greatly.

Word Problem:

Suppose a CPU is rated at 2.5GHz. what is the average length of time needed to execute one instruction?

Solution:

2.5GHz is 2.5 X 10exp9 instructions per second. That is 1/2.5 X 10exp9 = .4*10exp-9 or 400 picoseconds per per instruction.

1. Why is 1 being divided into 2.5

2. I don't get 400 picoseconds when I calculate 4*10exp-9

Can someone please help?