He pays $450 in taxes.
Since the first $10,000 of the assessed value is exempt, we subtract $10,000 from the $25,000 assessed:
25000-10000=15000
Now we need to see how many $100-increments are in 15000. We do this by dividing:
15000/100 = 150
Each of these raises the taxes by $3:
150(3) = 450