Question

pipe a can empty a filled tank in 28 hours while pipe b can fill the same tank when empty in 35 hours. if alternatively pipes a and b are turned on for an hour each time,starting when the tank is full how long will it take to empty the tank?

Answer 1

1/35 -1/28=- 7/(35*28)
i.e -1/140
so 5th part will be emptied in 1 he
next hr 4th part will be filled again
so in 2 hrs 1th part of the tank will be filled
till 135th part and 270 hrs it is all the way same
but after it., a will empty the tank in 1hr
so Ans= 271 hrs

Answer 2

Step-by-step explanation:

Pipe A can empty a filled tank in 28 hours,

⇒ Pipe A 1 hour's work = 1/28

while Pipe B can fill the same tank, when empty, in 35 hours,

⇒ Pipe B 1 hour's work = 1/35

⇒ Pipe (A + B) 1 hour’s work = 1/28 – 1/35 = 1/140

5th part will be emptied in 1 hour and next hr 4th part will be filled again.

Then, in 2 hrs 1 part of the tank will be filled,

For 135th part, in 270 hrs, it is filled as same.

Then, pipe A will empty the tank in 1hr.

∴ Total hours = 271 hours