9. the average annual salary for a postsecondary math
teacher is $21,760 less than that of a mathematician in
industry. together the average salaries total $134,720.
find each average annual salary.

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