lets do equal fractions by multiplying and dividing them by a number:
(2/5)(3/3) = 6/15
= (6/15)(3/3) = 18/45
so these are equivalent:
2/5 = 6/15 = 18/45
The fraction 12/15 is not equivalent to them, because there is no number that multiplying and dividing it gives the others.