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