Question 970736
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{{{20t-5t^2=15}}}
{{{20t=5t^2+15}}}
{{{0=5t^2-20t+15}}}
{{{0=t^2-4t+3}}}
{{{0=(t-3)(t-1)}}}
{{{t-3=0}}} {{{or}}} {{{t-1=0}}}
{{{t=3}}} {{{or}}} {{{t=1}}}
ANSWER: 
The particle reaches 15 meters on the way up after 1 second,
and passes 15 meters again after 3 seconds on the way down.