Question 623914
Call the distance from a to b {{{ d }}} miles
If there were no current at all, the  boat 
would cover {{{ d }}} in {{{ t = d/7 }}} hours
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In {{{ d/7 }}} hours, the current would have moved
the boat downstream {{{ 2(d/7) }}}
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The boat would be 2/7 of the distance a-b
from b downstream