Question 99649
total number of fish = x

I assume the 50 fish they caught were necessarily different. Maybe this assumption is unwarranted.

{{{x-100/x = 3/50}}}
{{{50x-5000 = 3x}}}
{{{47x = 5000}}}
{{{x = 5000/47}}}
{{{x = 106}}} (approximately)