jmc

First we note that no prime number is abundant since the sum of the proper factors of any prime is 1. Analyzing the remaining numbers, we find that 12 ($1+2+3+4+6=16>12$), 18 ($1+2+3+6+9=21>18$), 20 ($1+2+4+5+10=22>20$), and 24 ($1+2+3+4+6+8+12=36>24$) are abundant numbers. Thus, $\boxed{4}$ numbers less than 25 are abundant numbers.