Question 719065
#64 and #65 are really the same exact problem.  The description or question is incomplete.  What is needed is information about how much total rock.  Question asks how many tons of each rock but specification on the question is for only a rate. Not about how much total tons of rock.  


If x = tons of lower oil rock and y = tons of higher oil rock, then
{{{(19x+30y)/(x+y)=23}}} gallons of oil per ton of rock.


Not much more can be done.  Maybe get the linear equation simplified but still it has two unknown variables.