Volume of dam
$V = A_bL$
$V = [ \, \frac{1}{2}(4 + 12)(15) + 4(4) \, ](50)$
$V = 136(50)$
$V = 6800 ~ \text{ft.}^3$
$W = \gamma V$
$W = 150(6800)$
$W = 1\,020\,000 ~ \text{lb}$
$W = 1\,020\,000 ~ \text{lb} \times \dfrac{1 ~ \text{short ton}}{2000 ~ \text{lb}}$
$W = 510 ~ \text{short tons}$ answer