To find the rate of the second bus, we can solve the equation that has been given:
\[ 3(r + 10) + 3r = 282 \]
This equation represents the distance traveled by both buses in 3 hours. The first bus travels at a speed of \( r + 10 \) (10 mph faster than the second bus), and the second bus travels at a speed of \( r \).
Let's simplify the equation:
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Distribute the 3: \[ 3(r + 10) + 3r = 282 \] becomes \[ 3r + 30 + 3r = 282 \]
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Combine like terms: \[ 6r + 30 = 282 \]
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Subtract 30 from both sides: \[ 6r = 252 \]
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Divide by 6: \[ r = 42 \]
Thus, the rate of the second bus is \( \boxed{42} \) mph.