Explanation

(b) Here : Energy of one apple $= 21\ KJ = 21 \times 10^{3}\ J$
Efficiency of the boy $= 28\% = 0.28$
Mass of the boy $m = 40\ kg$
Here the actual energy consumed by the boy is given by as
$0.28 \times 21000 = 5880\ J$ …..(i)
And the energy consumed by the boy in climbing $h\ $meter height is given by
$= m\ g\ h = 40 \times 9.8 \times h$ …(ii)
Equating equations (i) and (ii) we get
$40 \times 9.8 \times h = 5880$
$h = \frac{5880}{40 \times 9.8} = 15\ m$