It is a compiler error to subtract two pointers that are referencing different arrays.

It is a compiler error to use pointer arithmetic with a pointer that does not reference an array.

Adding  to a pointer increases the address stored in the pointer by  byte.

Given the definitions

int array[1000];
int *pa;

is the expression pa = array valid?

The partial pressure of O₂ in the atmosphere at sea level is 21.3 kPa. What is the partial pressure of O₂ on the top of a mountain, where the atmospheric pressure is 65.1 kPa?

[Atmospheric pressure at sea level is 101.3 kPa.]