Machine A produces bolts at a uniform rate of 120 every 40 seconds and Machine B produce bolts at a uniform rate of 100 every 20 seconds. If the two machines run simultaneously, how many seconds will it take for tem to produce a total of 200 bolts.

Machine A:

In 40 seconds produces 120 bolts

In 1 second produces 120 / 40 bolts = 3 bolts

Machine B:

In 20 seconds produces 100 bolts

In 1 second produces 100 / 20 bolts = 5 bolts

So,

In 1 second with both machine simultaneously working,

Produces = 3 + 5 bolts = 8 bolts

Thus,

8 bolts is produced in 1 second

200 bolts is produced in (1 * 200) / 8 seconds

                                    = 25 seconds