It drains at a constant rate using the first line find the rate per hour:
-3.5/2 = -1.75 per hour
Now to find the missing totals multiply the hours by the rate:
-1.75 x 9 = -15.75
-1.75 x 23 = -40.25
Now to find the change between the two times subtract:
-40.25 - -15.75 = -24.50
