Question 713067
Variables to use:
u grams of the low concentrated alloy
v grams of the high concentrated alloy


The low concentration alloy, alloy L, is 5% gold and 10% lead.  High concentration alloy, alloy H, is 30% gold and 40% lead.


System to solve
{{{u*5+v*30=39}}}, for the grams gold result
{{{u*10+v*40=68}}}, for the grams lead result


The system would be easiest to solve if multiply the gold 39 equation by 2, and then subtract the lead 68 equation from it.  This would very quickly give a value for v.