Question 1193445
c speed of current
r speed of boat if no current


{{{system(r+c=210/10,r-c=210/70)}}}

.

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{{{system(r+c=21,r-c=3)}}}

add:
{{{2r=24}}}
{{{highlight(r=12)}}}


subtr:
{{{2c=18}}}
{{{highlight(c=9)}}}