Explanation :
Terzaghi's bearing capacity equation
$\begin{array}{l}{\mathrm q}_\mathrm u={\mathrm{CN}}_\mathrm c+{\mathrm{qN}}_\mathrm q+\frac12{\mathrm{ByN}}_\mathrm y\\\end{array}$
For cohensionless soil C = 0, Footing resting on sand surface so, D_{f} = 0
When water level is at base of footing
$\begin{array}{l}{\mathrm q}_{\mathrm u1}=\frac12{\mathrm{By}}_\mathrm{sub}{\mathrm N}_\mathrm y\\\end{array}$
When water level is at much greater depth
${\mathrm q}_{\mathrm u2}=\frac12{\mathrm{By}}_t{\mathrm N}_\mathrm y$
Percentage reduction in bearing capacity
$\begin{array}{l}=\frac{{\mathrm q}_{{\mathrm u}_2}-{\mathrm q}_{{\mathrm u}_1}}{{\mathrm q}_{{\mathrm u}_1}}\times100\\=\frac{{\mathrm y}_1-{\mathrm y}_\mathrm{sub}}{{\mathrm y}_\mathrm t}\times100\\\\\end{array}$
As ${\mathrm y}_\mathrm{sub}\simeq0.5\;{\mathrm y}_\mathrm t$
So, percentage reduction = 50%.