Answer: 4 second
To answer this question you need to know the difference between the cheetah and the antelope speed.
It would be: 90 feet/s- 60feet/s= 30 feet/s
That means the distance between the cheetah and the antelope will shorten 30 feet every second. After that divide the distance with the speed difference:
120 feet / ( 30feet/s )= 4 second