So, in order to calculate this we must find a quantity of each cream that we need. We need #x# gallons of cream 1 (21%) and #y# gallons of cream 2 (15%) and the total amount is 60 gallons (at 19%). That will look, as an equation, like this:
#0.21x+0.15y=0.19*60=11.4#
Since we know that the total is 60 gallons, we know that the sum of #x# and #y# is 60:
#x+y=60#
Now, if we solve the second equation for #y# (it would work the same if we solved for #x#) and plug it into the first equation, we can solve for a number value, like this:
#y=60-x#
#0.21x+0.15(60-x)=11.4#
#0.21x+0.15*60-0.15x=11.4#
#0.21x-0.15x+9=11.4#
#0.06x=11.4-9#
#0.06x=2.4#
#x=2.4/0.06=40#
So, if #x# is equal to 40, #y# must be equal to:
#y=60-x#
#y=60-40=20#
