let x be the number of gallons of 4% butterfat and y be th enumber of gallonns of 1% butterfat

set up two equations:

.04x + .01y = .02(18)

which simplifies to 

.04x + .01y = .36

the other equation is

x + y = 18 since you need 18 gallons total. solve for x in this by subtracting y from both sides

x = 18 - y

substitute this in to th eother equation for x

.04(18 - y) + .01y = .36, simplify and solve for y

.72 - .04y + .01y = .36

.72 - 03y = .36 subtract .72 from both sides

-.03y = -.36 divide both sides by -.03

y = 12

substitute this in to solve for x

x = 18 - y

x = 18 - 12

x = 6

so you need 6 gallons of 4% butterfat milk and 12 gallons of 1% butterfat milk

***if you need further explanation or have any questions feel free to message me!! :)