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One pipe can fill a pool in 9 hours. Another pipe can fill the pool in 6 hours. How long would it take them to fill the pool if they were working together?

3.6 hours

Step-by-step explanation:

To determine the time needed if they are working together, we use the formula

1/Ta + 1/Tb = 1 Tc

where Ta is the the time for the first item working alone

Tb is the time for the second item alone

Tc is the time for them working together

1/9 + 1/6 = 1/Tc

I multiply by 54Tc  to clear the fractions

54Tc (1/9 + 1/6) = 1/Tc *54Tc

6Tc + 9 Tc = 54

Combine like terms

15Tc = 54

Divide each side by 15

15 Tc /15 = 54/15

Tc =3.6 hours

3.6 hours

Step-by-step explanation:

Let X be the total job

Combined speed = X/9 + X/6

LCM = 18

(2X + 3X)/18

5X/18 = X/T

T = 18/5

T = 3.6