<PRE><B>Question 139700</B>
Let {{{h}}}= km walked on hills
Let {{{f}}}= km walked on flat groun
{{{h + f = 18}}}
{{{h/3 + f/4 = 5}}}
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{{{f = 18 - h}}}
Use substitution
{{{h/3 + (18 - h)/4 = 5}}}
Multiply each side by 12
{{{4h + 3*(18 - h) = 60}}}
{{{4h + 54 - 3h = 60}}}
{{{h = 6}}}km
{{{f = 18 - h}}}
{{{f = 18 - 6}}}
{{{f = 12}}}km
6 km of the trip was over hills
check answer:
{{{h/3 + f/4 = 5}}}
{{{6/3 + 12/4 = 5}}}
{{{2 + 3 = 5}}}
OK</PRE>