Question

The braking distance, D, of a car is directly proportional to the square of its speed, v. When d=5, v=10

Answer 1

$d \propto v^2$

$\implies d=kv^2$

substitute the given values, $5=k(10)^2\implies k=\frac1{20}$

now, $d=\frac{1}{20}\times( 70)^2=\frac{70\times70}{20}=245$

A big blunder from my side, now fixed!