types and pointers address

Suppose the first element of double u[100] has address 0x1000, and sizeof(double) is 8. What does

printf("%p %pn", &u[3] - 1, &u[20] - 5)

output?

The homework answer is

echo '0x1010 0x1078'      

How did they do the second part &u[20] - 5?