Hi Barbara
Let's define some variables
P = gallons of paint
T = gallons of thinner
P + T = 16 We will have a mixture of a total of 16 gallons
19P + 3T = 14(16) The cost of paint if $19/gal and the cost of the thinner is $3/gal. Together we have a mixture of 16 gallons at $14/gallon
We have a systems of equations, which you can solve
P + T = 16
19P + 3T = 14(16)
19P + 3T = 14(16)