Question

In a computer game, there are builders and destroyers. Together there are 20 of them.
Some of them try to build a wall around a castle while the rest try to demolish it. Each of
the builders can build the wall alone in 15 hours while any of the destroyers can demolish
it in 10 hours. If all 20 builders and destroyers are made active when there is no wall and
the wall gets built in 3 hours, how many of them are destroyers?​

Answer 1

Answer:

9 Destroyers

Step-by-step explanation:

Destroyer + Builder = 25

Let say Number of builders = B

Then Number of destroyer = 25 - B

1 Builder can build wall in 20 hrs

in 1 hr  1 builder can built wall  = 1/20

B builders in 1 hr can build wall in B/20

B builders in 5 hr can build wall in 5B/20  = B/4

1 Destroyer  can demolish wall in 15 hrs

in 1 hr 1 destroyer can demolish wall  = 1/15

25-B destroyers in 1 hr can demolish wall  = (25 - B)/15

25-B destroyers in 5 hrs  can demolish a  wall in  5(25-B)/15  =  (25-B)/3

Wall built - wall demolished = 1

=> B/4  - (25-B)/3  = 1

multiplying by 12 both sides

=> 3B - 100 + 4B = 12

=> 7B = 112

=> B = 16

16 Builders are there

25-16 = 9 Destroyers are there

Answer 2

Step-by-step explanation:

x/15- (20-x/10)= 1/3

solve it...

x= 14

now

destroyer = 20-14= 6