Đáp án:

$560\, km$

Giải thích các bước giải:

We have the ratio of speed of two cars was $4:3$

Let $4x,3x$ be the speed of Ford and Audi from the beginning, respectively $(x > 0)$

$\to 4x.0,75 = 3x$ was the speed of Ford after it met Audi

$\to 3x.1,25 = 3,75x$ was the speed of Audi after it met Ford

With $t$ was the time they started leaving till they met each other.

And $t'$ was the time they ran after passed each other.

The distance $(s)$ between Hanoi and Nghe An is:

$s = 4xt + 3xt = 7xt\qquad (1)$

$s = 4xt + 3xt'\qquad \qquad \,\,\,\,(2)$

$s = 3xt + 3,75xt' + 20\quad\, (3)$

$s = 3xt' + 3,75xt' + 20\quad (4)$

From $(1)$ and $(4)$ we get:

$7xt - 6,75xt' = 20 \quad (*)$

From $(2)$ and $(3)$ we get:

$xt - 0,75xt' = 20$

Therefore: $6xt = 6xt'\Leftrightarrow t = t'$

Thus, from $(*)$ we get: $0,25xt = 20$

$\Leftrightarrow xt = 80$

$\Leftrightarrow 7xt = 560$

$\Leftrightarrow s = 560$ km