Which expression is the best estimate of 4/5 x 22.01?

The answer is:
Justin weighs 165 pounds.
Greg weighs 180 pounds.

If:
j - Justin's weight
g - Greg's weight

Then the system of equations is:
j + 15 = g              ⇒ g = j + 15
1/2g = j - 75          ⇒ j = 1/2g + 75

We can replace j in the first equation:
g = 1/2g + 75 + 15
g - 1/2g = 90
1/2g = 90
⇒ g = 90 ÷ 1/2 = 180

Thus, Greg weighs 180 pounds.

Now, using the first equation, we will calculate Justin's weight:
j + 15 = g                    ⇒ j = g - 15
g = 180

Thus 
j = g - 15 = 180 - 15 = 165

Therefore, Justin weighs 165 pounds.