Question 910581
What you want is- if you know Mark's height,
you can find Debbie's height and vice  versa
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Let {{{ t }}} = time in minutes starting when Mark is at
12,000 ft and Debbie is at 6,000 ft
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Mark's equation:
{{{ h[m] = 12000 - 250t }}}
Debbie's equation:
{{{ h[d] = 6000 + 60t }}}
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{{{ Let {{{ D = h[m] - h[d] }}}
{{{ D = 12000 - 250t - ( 6000 + 60t ) }}}
{{{ D = 12000 - 250t - 6000 - 60t }}}
{{{ D = 6000 - 310t }}}
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If you know one of their heights, you can find the other