Three numbers are chosen at random.

the middle number is 4 more than the smallest number.
the largest number is two times the smallest number.
the sum of the 3 numbers is 54.

what is the smallest number?

So we have x,y,z
smallerst=x
middle=y
z=largest

so
middle is 4 more than smallest
y=4+x

largest is 2 time smallest
z=2x

sum (addition ) of the three is 54
x+y+z=54

so subsitute
z=2x
y=x+4
x=x

therefor
x+y+z=54
x+x+4+2x=54
add like terms
4x+4=54
subtract 4
4x=50
divide both sides by 4
x=25/2=12.5

the smallest number is 12.5