Question 539840
r*t=d
90*t=100+x
70t=x
90t=70t+100
20t=100
t=5 seconds
after one second the cheetah has gone 90 feet and the gazelle has gone 70 more
now the distance is 80. 
So every second the cheetah gains 20 feet

450 feet for the cheetah 
350 for the gazelle plus the original 100 feet.