It would take one machine $4$ hours to complete a production order and another machine $2$ hours to complete the same order. If both machines work simultaneously at their respective constant rates, the time taken to complete the same order is ________ hours.
Let the production order be 8 items(any multiple of L.c.m(2,4) like 4,8,12,16... would work).
Since 1 st machine completes the order in 4 hours, it is producing at a rate of 2 units per hour.
Similarly 2 nd machine produces at a rate of 4 units per hour.
If both machines are working simultaneously, together they produce 6 units per hour.
$\therefore$ 8 units are produced in $8/6$ hours.
option c is the answer