Which of the following must be true in order for an object to stay in place? the

The forces must be balanced.

This is in accordance to Newton's first law of motion, which states that an object at rest will remain at rest and an object in motion will remain in motion unless acted upon by an unbalanced force.
An unbalanced force is also known as a resultant force.

480,026ns

Explanation:

the time it would take to access word = 20ns

time coipied in cache = 60 ns

20ns +60ns = 80ns

this is the time to acess from main memory

it takes 12ms to move memory word to main memory

1ms is 1000000

so 12ms = 12000000ns

1200000+20+60

= 12000080 nanoseconds

hit ratio = 0.9(cache)

hit ratio = 0.6 (main memory)

miss rate = 0.1

probability of word being in main memory

= 0.1 * 0.6

= 0.06

the hit ratio of disk = 1

prob = 0.1 * 0.4 * 1 = 0.04

.1 = miss rate of cache

.4 = miss rate of main memory

1 = hit ratio of hard disk

In cache with prob 0.9, total ns = 20

in main memory prob = 0.60, total ns = 80

in hard disk prob = 0.04, total ns = 12000080ns

average time = [pr(cache)*access time] + [pr(main memory)*access time] + [pr(hard disk)*access time]

= (0.9*20)+(0.06*80)+(0.04*12000080)

= 18+4.8+480,003.2

= 480,026 ns