Question 318904
Let P be the plane speed.
1.{{{(P+20)t=450}}}
2.{{{(P-20)t=300}}}
From eq. 1,
{{{t=450/(P+20)}}}
Substitute in eq. 2,
{{{(P-20)*(450/(P+20))=300}}}
{{{(P-20)*45=(P+20)30}}}
{{{45P-900=30P+600}}}
{{{15P=1500}}}
{{{highlight(P=100)}}} mph