A large truck, traveling 30 miles per hour, carries a letter from the central post office to the local post office. The letter is then loaded onto a local vehicle, which travels at an average speed of 10 miles per hour, until the letter reaches its destination. The large truck carried the letter for a minutes and the local vehicle carried the letter for b minutes. If the total distance that the letter travelled from the central post office to its destination was 24 miles, which of the following equations correctly relates a and b?​