15/[1-(1/4)-(1/4)*(4/3)]
=15/(5/12)
=36 吨
方程法：设黄沙总共有x吨
(1/4)x+(1/4)x*[1+(1/3)]+15=x
(1/4)x+(1/3)x+15=x
(7/12)x+15=x
(5/12)x=15
x=15*12/5
=36
答：黄沙总共有36吨