If Bob has half of a pumpkin pie. He has 50% of the pie. So if you ask for a peice and he gives you half of it, you have 1/4 of a pie, or, 25%.  23% is 23/100.        you take the number and put  it over 100.    2.5= 2.5 over 100.  

1/4 = 25/100 =25%. 

Horselover