## A can build a wall in 30 days. B can demolish that wall in 60 days. If A and B work on alternate days, when will the wall be completed?

## a) 180 days b) 90 days c) 120 days d) 60 days

## Answer is : C

## Solution :

**Given :-**

1. A build in => 30 days

2. B demolish in => 60 days

**What To Find:-**

How much time will it take to build the wall if A and B work in Alternative days?

**Procedure :-**

1. in 1 day A can build = 1/30 th part

2. in 1 day B can be fill = -1/60 th part

So,

for first 2 days A and B can build = 1/30 - 1/60 = 1/60

=> In 2 minutes A+B+C can be build = 1/60th Part.

=> Time taken to build 1/60 part = 2 days

=> Time taken to build full part = 2 * 60 days = 120 days.

(Note :- in Problem it is clear that first

A will work for 1 day and then

B will work for 1 day

and again A and B and so on......
So for first 2 days A and B will work each for one day)

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