Question 1112634
Half hour means  {{{1/2}}} hour.
15 minutes means  {{{1/4}}} hour.


Basic rule for constant travel rates is  {{{(RATE)(TIME)=DISTANCE}}}.


Your problem describes the sum of two distances.  That sum is given or described as HALF of a full distance.


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{{{3*(1/2)+7(1/4)=d/2}}}

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{{{3+7/2=d}}}

{{{6/2+7/2=d}}}

{{{13/2=d}}}

{{{highlight(d=6&1/2)}}}