Answer:
10 inches
Explanation:
You want to know how much Jack's plant grew in 140 days at the rate of 50 inches in 700 days.
Proportion
The constant growth rate means the growth is proportional to the days:
height/days = h/140 = 50/700
h = 140(50/700) = 10
Jack's plant grew 10 inches in 140 days.