To find the rate of the second bus, we start with the equation given:
\[ 3(r + 10) + 3r = 282 \]
First, distribute the 3 to both terms in the first part of the equation:
\[ 3r + 30 + 3r = 282 \]
Combine like terms:
\[ 6r + 30 = 282 \]
Next, subtract 30 from both sides:
\[ 6r = 252 \]
Now, divide both sides by 6 to solve for \( r \):
\[ r = 42 \]
Therefore, the rate of the second bus is
\[ \boxed{42 \text{ mph}} \]