[tex]\qquad\qquad\huge\underline{{\sf Answer}}[/tex]
Here we go ~
The air plane descends 15 feels every 1.45 seconds, so it's unit rate is :
[tex]\qquad \sf \dashrightarrow \: \dfrac{15}{1.45} \: \: ft / sec[/tex]
Now we have to calculate the descent in 3.5 minutes ~
[tex]\qquad \sf \dashrightarrow \: \dfrac{15}{1.45} \: \: \times 3.5 \times 60[/tex]
[tex]\qquad \sf \dashrightarrow \: 2172.41 \: \: ft[/tex]
So, the airplane descends 2172.41 feels in three and a half minutes ~