A car covers the first 39 kms of its journey in 45 minutes and covers the remaining 25 km in 35 minutes. What is the average speed of the car? (Bank P.O., 2007)

### Concept & Average Speed Average speed over multiple segments is always the sum of all distances divided by the sum of all time periods. Ensure time units are consistent with the requested speed units (km/hr requires time in hours). $$Average\ Speed = \frac{Total\ Distance}{Total\ Time}$$ ### Step-by-Step Solution * **Step 1: Calculate Total Distance.** * Total Distance $= 39$ km $+ 25$ km $= 64$ km. * **Step 2: Calculate Total Time in hours.** * Total Time in minutes $= 45$ mins $+ 35$ mins $= 80$ minutes. * Convert minutes to hours: $\frac{80}{60}$ hours $= \frac{4}{3}$ hours. * **Step 3: Calculate Average Speed.** * Average Speed $= \frac{64}{4/3}$ * Average Speed $= 64 \times \frac{3}{4}$ * Average Speed $= 16 \times 3 = 48$ km/hr. ### Exam Strategy & Shortcut Instead of converting $45$ mins and $35$ mins to hours individually (which leads to adding fractions $\frac{3}{4} + \frac{7}{12}$), sum the minutes first to get $80$ minutes, and then perform a single, much simpler conversion to $\frac{4}{3}$ hours. ### Common Pitfall Failing to convert the total time from minutes to hours before dividing, which would result in $\frac{64}{80} = 0.8$, a non-sensical speed for a car in this context. ### Final Answer Therefore, the correct answer is **48 km/hr**.