The volume of water leaked from the tank from 2:30 pm to 3:25 pm is the difference

       -  = simplify using common denominator =  =  = 


of the total tank volume.  The time elapsed from 2:30 pm to 3:25 pm is 55 minutes.


So, every 5 minutes, the leaking volume is    of the tank volume.


At 3:25 pm the tank is/was   =   full.


Hence, the tank will be empty in 5*10 = 50 minutes after 3:25 pm, which is 4:15 pm.    ANSWER

To make the fractions easy to use, suppose the tank holds 35 gallons, or
35 of any unit of volume you choose.

At 2:30, the tank contains 21 gallons. (3/5 of 35)
55 minutes later (at 3:25) the tank contains 10 gallons. (2/7 of 35).

So it leaked out 11 gallons in 55 minutes. (21-10)
How long will it take to leak out those remaining 10 gallons?

Set up the proportion: (gallons over minutes equals gallons over minutes):







So 50 minutes after 3:25 is 4:15. 

----------------------------------------

Here's an alternate way to get the answer without changing any of the
original information (not the easiest way! LOL)

Let x = the number of hours after 12 noon or midnight
Let y = the fraction of fullness the tank is at time x.

We want to know what x is when y = 0.

When its  hours past 12 noon or midnight, it's ths full.
When its  hours past 12 noon or midnight, it's ths full.

So we want the equation of the line through the points:

 and 
which is
 and 
which is
 and 

Slope formula:

where (x1,y1) = 
and where (x2,y2) = 


Multiply top and bottom by 



Point-slope formula:

where (x1,y1) = 


Substituting 0 for y, to find when tank is empty:



Multiply through by 35












So the tank became empty at 4 1/4 hours after 12 noon or midnight.

1/4 of an hour is 15 minutes

So the instant the tank became empty was at 4:15 PM or AM

Edwin