72 minutes

no of products manufactured by machine I & II in 1 hr or 60 mns=100+150=250
to make 300 products therefore they would take 60/250*300=72 minutes

Notice that, if both machines work at the same time, they make 250 pieces in one hour. This means that we need to find the time it takes for both machines to make 50 pieces.
Again, notice that the second machine makes 50% more pieces in the same time. So let's say:
a = number of pieces made by the first machine
b = number of pieces made by the second machine = 1.5*a
You get that a + 1.5a = 50 or that 2.5a = 50, with a = 20.
To find out the time it takes machine 1 to make 20 pieces, we need to remember that it takes 1 hour to make 100 pieces. This means that it will take 20/100 or one fifth of an hour to make 20 pieces.

So the whole thing will take 1 hour and 12 minutes. or 72 minutes.

deepoe-

This should be a simple work problem. Remember,
1/x + 1/y ... = completes/time

Usually, you see these problems as "John completes a job in 5 hours, Mary completes the same job in 2 hours..."

Or, you might see, "Machine A completes a job in... Machine B completes a job in..."

Same concept here.
1/(1/100) + 1/(1/150) = 250/1 or 250 units produced/hour.

We know that 300 units is a 20% increase over 250 units (or 1/5 increase). So, it should take 1 hour + (1 hr)(1/5) =
1 hr + 1/5 hr or 1 hr 12 min.

Quickly converted, that's 72 minutes.