Machine A produces a batch in 3 hours; machine B produces the same batch in 2 hours. How long do they take together?

Rates add when machines work in parallel: $1/3 + 1/2 = 1/x$. Common denominator 6: $2/6 + 3/6 = 5/6$, so $1/x = 5/6$ and $x = 6/5 = 1.2$ hours, that is 1 hour 12 minutes. Option A wrongly adds the individual times; option B halves that sum.