Correct Answer - Option 2 : 74%
Concept:
Bracking efficiency is given by,
\(\eta = \frac{{{V^2}}}{{2gfl}}\)
Where, V = Velocity of vehicle
l = length of skidding
f = average skid resistance
η = brake efficiency
g = acceleration Due to gravity
Calculation:
\(V = 40\;km/h = \frac{{100}}{9}m/s\)
l = 12.2 m
f = 0.70
\(\eta = \frac{{{{\left( {\frac{{100}}{9}} \right)}^2}}}{{2 \times 9.81 \times 0.70 \times 12.2}} = 0.736 = 73.6\% \simeq 74\%\)
