One hydrant can fill a truck in 12 minutes, another in 6 minutes . If they are both turned on together how long would the truck take to be filled?

12+6= 18\2= 9 minutes

first one fills 1 truck /12 min

second fills 1 truck/6 min
so
together they fill (1/12 + 2/12) trucks per min
which is
1/4 of a truck per minute
or
four minutes per truck.
Get it?