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Explanation:

The ratio QR : RS : ST  is equal to 1 : 4 : 5

This means we have the following three equations

For some positive number x.

Note the ratio QR : RS : ST turns into x : 4x : 5x, which reduces to 1 : 4 : 5 when we divide all three parts by x .

Along with those three equations, we'll also use QT = 40 as well.

Now turn to the segment addition postulate. Plug in the equations mentioned earlier, and solve for x .

QT = QR + RS + ST

40 = x+4x+5x

40 = 10x

10x = 40

x = 40/10

x = 4

So we know that

QR = x = 4

RS = 4x = 4*4 = 16

ST = 5x = 5*4 = 20

As a check,

QR + RS + ST = 4 + 16 + 20 = 40

which is the same as QT = 40

Therefore, we've confirmed that QR + RS + ST = QT is correct and we've confirmed our answers.

12r - 8 - 12

you have to combine like terms. below i have bolded the like terms:

12r - 8 - 12

together it combines to:

12r - 20

hope this !