A student first rode a bus traveling at an average speed of 30kph and then took a taxi traveling at 60kph. The entire distance was 150 km and the entire trip to 3hrs. How far did he travel riding the taxi?
Let distance traveled by taxi be D
Then distance traveled by bus = 150 - D
We then get the following TIME equation: 
First, multiply the equation by its LCD, 60, and then continue solving for D, the distance traveled by taxi.
That's all! NOTHING more, NOTHING less!!