🗺️ Chapter Roadmap
1 The DST Triangle
Every problem involving travel is based on three things: Distance, Speed, and Time. To remember all three formulas easily, use this triangle:
2 ⚡ The "5 / 18" Magic Trick
Exam questions often give the Speed in km/hr but ask for the Time in seconds. You must convert the units properly, or your answer will be wrong.
🚀 km/hr to m/s
Going from Big to Small?
Multiply by 5/18
🐢 m/s to km/hr
Going from Small to Big?
Multiply by 18/5
3 ⚠️ The Average Speed Trap
If a person travels from point A to B at speed x and returns from B to A at speed y, the average speed is NOT (x + y) / 2. Use this formula instead:
Simple Average = (x + y) / 2 (WRONG!)
Works only when the distance traveled for both speeds is the same.
4 Relative Speed (Overtaking)
When two objects are both moving at the same time, we calculate their "Relative Speed" based on their direction:
Opposite Direction (Towards each other)
Speeds are Added (+). They will meet faster!
Same Direction (Chasing / Overtaking)
Speeds are Subtracted (-). It takes longer to catch up.
5 Train Problems (The Rule of Distance)
In train problems, the "Distance" is not always given. You have to logic it out:
Since a pole has almost zero thickness, the Distance = Length of the Train itself.
The train has to cover its own length AND the platform length.
Distance = (Length of Train + Length of Platform).