From CBS News:

The military campaigns in Iraq and Syria have taken 45,000 enemy combatants off the battlefield and reduced the total number of Islamic State of Iraq and Syria (ISIS) fighters to as few as 15,000, the top U.S. commander for the fight against ISIS said Wednesday.

Lt. Gen. Sean MacFarland said that both the quality and number of IS fighters is declining, and he warned that it is difficult to determine accurate numbers. Earlier estimates put the number of ISIS fighters between 19,000 and 25,000, but U.S. officials say the range is now roughly 15,000 to 20,000.

When you take the previous highest estimate and subtract the current lowest estimate, you get

25,000 – 15,000 = 45,000

Yep!  It all checks out!