Question 1101062
Think of this as one of them standing still
and the other one moving at the sum of their speeds
Let {{{ s }}} = the speed of the slower cyclist
{{{ 2s }}} = the speed of the faster cyclist
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{{{ 90 = ( s + 2s )*2 }}}
{{{ 90 = 3s*2 }}}
{{{ 90 = 6s }}}
{{{ s = 15 }}} 
and
{{{ 2s = 30 }}}
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Their speeds are 15 km/hr and
30 km/hr
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check:
{{{ d[1] = 15*2 }}}
{{{ d[1] = 30 }}} km
and
{{{ d[2] = 30*2 }}}
{{{ d[2] = 60 }}} km
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{{{ d[1] + d[2] = 30 + 60 }}}
{{{ d[1] + d[2] = 90 }}} km
OK