Question 393236
Let {{{d}}} = the distance covered at 80 km/hr
Then {{{690 - d}}} = distance covered at 90 km/hr
Let {{{t}}} = the time in hrs driven at 80 kmm/hr
The {{{8 - t}}}  = time in hrs driven at 90 km/hr
I can write 2 equations:
(1) {{{d = 80t}}}
(2) {{{690 - d = 90*(8 - t)}}}
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From (1):
{{{t = d/80}}}
and, substituting:
(2) {{{690 - d = 90*(8 - d/80)}}}
{{{690 - d = 720 - (9/8)*d}}}
{{{5520 - 8d = 5760 - 9d}}}
{{{d = 240}}}
 The distance that he covered at 80 km/hr was 240 km
check:
{{{t = d/80}}}
{{{t = 240/80}}}
{{{t = 3}}} hrs
(2) {{{690 - d = 90*(8 - 3)}}}
{{{690 - 240 = 90*5}}}
{{{450 = 450}}}
OK