Let the second car overtake after ( n ) hours.

Distance by first car: ( 20n ) km.

Distance by second car: speeds ( 12,13,14,\ldots,(11+n) ) form an A.P. [ S_n = \frac{n}{2}\bigl[2\cdot 12 + (n-1)\cdot 1\bigr] = \frac{n}{2}(24 + n - 1) = \frac{n}{2}(n+23). ]

Set equal: [ \frac{n}{2}(n+23) = 20n. ] Assuming ( n\neq 0 ), [ \frac{n+23}{2} = 20 \implies n+23 = 40 \implies n = 17. ]

Answer: 17 hours