Answer :

To transform it into percent we want to have 100 into denominator. The easiest way is divide numerator and denominator by 3 (thats because than it doesnt change the value of fraction), so:
[tex] \frac{258}{300}= \frac{258:3}{300:3}= \frac{86}{100} = 86% [/tex] - its the result
[tex]258\ \ |\ \ :2\\129\ \ |\ \ :3\\43\ \ \ \ |\ \ :43\\1\\\\258=6\cdot43\\\\300\ \ |\ \ :2\\150\ \ |\ \ :2\\75\ \ \ \ |\ \ :3\\25\ \ \ \ | \ \ : 5\\5\ \ \ \ \ |\ \ :5\\1\\\\300=6\cdot50\\\\ \frac{\big{258}}{\big{300}} =\frac{\big{6\cdot43}}{\big{6\cdot50}} =\frac{\big{43}}{\big{50}} [/tex]

[tex]\\\\\frac{\big{43}}{\big{50}} =\frac{\big{43}}{\big{50}} \cdot 100\%=\frac{\big{43}}{\big{50}} \cdot 50\cdot2\%=43\cdot2\%=86\%[/tex]