Question 119292
Distance(D) = Rate(R) x Time(t)
Since one plane flies east and one plane flies west, add their distances together to find out how far apart they will be. 
Since they leave at the same time, t is the same for both, only R is different.
{{{D=R[1]*t+R[2]*t}}}
{{{2250=300*t+450*t}}}
{{{2250=750*t}}}
{{{t=3}}}
In 3 hours, they will be 2250 km apart.