$A$ is travelling at 72 km per hour on a highway while $B$ is travelling at a speed of 25 metres per second. What is the difference in their speeds in metres per second?

### Concept & Unit Standardization Before comparing or finding the difference between two speeds, they must be expressed in the exact same units. We will convert km/hr to m/sec using the standard multiplier. $$ \text{Speed in m/sec} = \text{Speed in km/hr} \times \frac{5}{18} $$ ### Step-by-Step Solution * **Given:** Speed of $A$ = $72$ km/hr, Speed of $B$ = $25$ m/sec. * **Convert A's speed:** * Speed of $A$ in m/sec $= 72 \times \frac{5}{18}$ * $= 4 \times 5 = 20$ m/sec. * **Calculate the difference:** * Difference = Speed of $B$ - Speed of $A$ * Difference $= 25 - 20 = 5$ m/sec. ### Exam Strategy & Shortcut Recognize common km/hr to m/sec conversions instantly. $18$ km/hr = $5$ m/sec, $36$ km/hr = $10$ m/sec, $54$ km/hr = $15$ m/sec, $72$ km/hr = $20$ m/sec. Memorizing these multiples of $18$ allows you to bypass the calculation step entirely and immediately see the difference is $25 - 20 = 5$. ### Common Pitfall Subtracting the raw numbers ($72 - 25$) without noticing the mixed units (km/hr vs m/sec). Always standardize units before performing arithmetic operations. ### Final Answer Therefore, the correct answer is **5 m/sec**.