A person of average (or even below average) ability and experience can figure out how to use the thing [i.e., it's learnable] to accomplish something [effective] without it being more trouble than it's worth [efficient].
A person of average (or even below average) ability and experience can figure out how to use the thing [i.e., it's learnable] to accomplish something [effective] without it being more trouble than it's worth [efficient].