Hi
Bunuel,
Can't we estimate 1 inch=0.0254 as 1 inch=0.02=1/50. I've calculated this way and got 45km p/h avr.speed and selected B. Please have a look at my solution below:
1)Circumference of the wheel: 2πr= 20 inch * (≈3)≈60 inch. Thus, in 1 revolution car goes 60 inch;
2)Calculating 1 revolution in meters: Since 1 inch=0.0254≈0.02=1/50 meters, then 60 inch=60*1/50=6/5=1.2 meters. Thus, in 1 revolution car goes 1.2 meters;
3)Now, we have to calculate in 75000 revolution how long, in meters, car would go: 75000*6/5=90,000 meters which is equal to 90 km.
4) Calculating avr. speed: Since in 2 hours car goes 90 km, then in 1 hour it should go 45km per hour. Hence, the closest answer is B- 41 kmp/h.
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