What is the correct way of calculating MIPS?

Question

I found below question about MIPS,

A computer system has a CPU with a word length of 64 bits and a clock speed of 1.5GHz. For a certain task, the measured average CPI (cycles per instruction) of the processor is 0.6. What is the MIPS rate of the processor?

The given answer is,

Instructions /second = cycles/second x instructions/cycles
= 1.5x10⁹x 1/0.6
= 2.5x10⁹/ 1 million
= 2500 MIPS

When I searched more examples, I found in most places the CPU speed was taken in Mhz (not as Hz) to calculate MIPS. Refer the below question & answer.

Alternatively, divide the number of cycles per second (CPU) by the number of cycles per instruction (CPI) and then divide by 1 million to find the MIPS. For instance, if a computer with a CPU of 600 megahertz had a CPI of 3: 600/3 = 200; 200/1 million = 0.0002 MIPS.

But, if I solve the second one as solved the first one, the answer would be,

= 600x10⁶x 1/3
= 200x10⁶/ 1 million
= 200 MIPS

What is the correct way of calculating MIPS?