You fill your car's gas tank when it's about 1/2 empty the next week you fill the tank and second time when it's about 3/4 empty if you buy a total of 18 and 1/2 gallons of gas on these two days about how many gallons does the tank hold

Let x=the number of gallons that the full tank holds 
The first time you fill it, you put (1/2)x gallons in 
The second time you fill it, you put (3/4)x gallons in 
Now we are told that (1/2)x+(3/4)x=18.5 gal or
(5/4)x=18.5 gal multiply both sides by 4 
5x=74 divide both sides by 5 

x=14.8 galnumber of gal the tank holds 
CK 
(1/2)*14.8+(3/4)*14.8=18.5 
7.4+11.1=18.5
18.5=18.5