Question 370206

{{{1/f=1/o+1/i}}}
{{{1/50=1/600+1/i}}}
{{{1/i=1/50-1/600}}}
Use a common denominator.
{{{1/i=12/600-1/600}}}
{{{1/i=11/600}}}
{{{highlight(i=600/11)}}}
.
.
.
{{{1/250=1/2000000-1/i}}}
{{{1/i=1/250-1/2000000}}}
{{{1/i=8000/2000000-1/2000000}}}
{{{1/i=7999/2000000}}}
{{{highlight(i=2000000/7999)}}}