If a 15.0 N force is applied to a level at a distance of 10.0 m from the fulcrum then how much force is needed at 25.0m from the fulcrum to balance the 15.0 N force?

Obviously, you want us to do your homework for you. That's a "No No". I think I'm done here.

Show us your work and we'll critique.

Find the anticlockwise and clockwise moments, from

Moment = Force \times Perpendicular Distance

The moments from both distances from the fulcrum have to be equal so that the system is balanced. So, equate your equations!