Which of the following trains is the fastest?

### Concept & Unit Equivalency To compare different speeds to see which is "fastest," you must convert all the given options into a single, common unit (e.g., meters per second) and compare their numerical values. $$ \text{km/hr to m/sec} = \times \frac{5}{18} $$ $$ \text{m/min to m/sec} = \div 60 $$ ### Step-by-Step Solution * **Analyze Option A:** The speed is already in m/sec. * Speed = $25$ m/sec. * **Analyze Option B:** Convert $1500$ m/min to m/sec. * Speed = $\frac{1500 \text{ m}}{60 \text{ sec}} = 25$ m/sec. * **Analyze Option C:** Convert $90$ km/hr to m/sec. * Speed = $90 \times \frac{5}{18} = 5 \times 5 = 25$ m/sec. * **Conclusion:** Options A, B, and C all represent the exact same speed ($25$ m/sec). Because they are equal, no single train is faster than the others. ### Exam Strategy & Shortcut Convert everything to m/sec because it is the standard SI unit and usually yields clean integers. As soon as you see two options calculate out to the exact same value in a "which is greatest" question with a "None of these" option, you know the answer must be "None of these". ### Common Pitfall Students might intuitively assume that the option with the largest raw number ($1500$ or $90$) is the fastest without standardizing the units first. ### Final Answer Therefore, the correct answer is **None of these**.