I live X floors above the ground floor of a high-rise building. It takes me 30 s per floor to walk down the steps and 2 s per floor to ride the lift. What is X, if the time taken to walk down the steps to the ground floor is the same as to wait for the lift for 7 min and then ride down?  Choose one.
A) 4	B) 7	C) 14	D) 15	E) 17
Explanation :   Since I live X floors above the ground floor and it takes me 30 s per floor to walk and 2 s per floor to ride, it takes 30X s to walk down and 2X s to ride down after waiting 420 s   =&gt; 30X = 2X + 420   =&gt; X = 15.   Answer : 15
<result>D</result>
I live X floors above the ground floor of a high-rise building. It takes me 30 s per floor to walk down the steps and 2 s per floor to ride the lift. What is X, if the time taken to walk down the steps to the ground floor is the same as to wait for the lift for 7 min and then ride down?