A nugget of gold and quartz was found to contain xg of gold and yg of quartz and has a density d. If densities of gold quartz are respectively d1 and d2, then
`x/d_1` + `y/d_2` = `(x + y)/d`
xd1 + yd2 = (x + y)d
`x/d_2` + `y/d_1` = `(x+ y)/d`
`(x + y)/d` + `x/d_1` + `y/d_2` = 0