If Machine X can produce 80 widgets in 4 hours, how long would it take machine X and Y working together, to produce 500 widgets?

==> X produces 20 widgets/hour (= 80/4)

A) Machine Y works at 3/4 the speed of machine X

==> Y produces 3/4 x 20 = 15 widgets/hour

==> (X + Y) produce (20 + 15) = 35 widgets/hour

==> to produce 500 widgets, (X+Y) need 500/35 hours

A is sufficient.

B)If Machine Y's speed doubled, it would produce 120 widgets in 4 hours

==> double speed of Y = 2 Y work simultaneously ==> (Y+Y) produce 30 widgets/hour (=120/4)

==> One Y produces 15 widgets/hour

==> (X + Y) produce (20 + 15) = 35 widgets/hour

==> to produce 500 widgets, (X+Y) need 500/35 hours

B is sufficient.

Answer is D.

Hope it helps.

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