If you want to check if two fractions are equivalent, you can use this Rule:
Two fractions (a/b and c/d) are equivalent only if the product (multiplication) of the numerator (a) of the first fraction and the denominator (d) of the other fraction is equal to the product of the denominator (b) of the first fraction and the numerator (c) of the other fraction.
In other words, if you cross-multiply (a/b = c/d) the equality will remain, i.e, a.d = b.c. So, here are some examples:
6/10 is equivalent to 3/5 once 6 x 5 = 10 x 3 = 30
9/15 is equivalent to 3/5 once 9 x 5 = 15 x 3 = 45
12/20 is equivalent to 3/5 once 12 x 5 = 20 x 3 = 60

More: 15/25, 18/30, 21/35, 24/40, 27/45, 30/50, and so on ...

So, 3/5 and 12/50 are not equivalent fractions.