Question 1031905
You can think of this as one of the planes
not moving and the other one traveling 
at the sum of their speeds
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Let {{{ t }}} = time in hours until
2nd plane intercepts 1st plane
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{{{ 200 = ( 350 + 450 )*t }}}
{{{ 200 = 800t }}}
{{{ t = 1/4 }}} hr
2nd plane intercepts 1st plane
in 15 min
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check:
1st plane:
{{{ d[1] = 350t }}}
{{{ d[1] = 350*(1/4) }}}
{{{ d[1] = 87.5 }}} km
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2nd plane:
{{{ d[2] = 450*(1/4) }}}
{{{ d[2] = 112.5 }}}
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{{{ d[1] + d[2] = 87.5 + 112.5 }}}
{{{ d[1] + d[2] = 200 }}} km
OK