You'd solve this equation by writing two equations. I'd call treadmills x and stationary bikes as y. The two equation would be x+y=25 and y=x+7. You'd then plug the second equation into the first equation to get x+(x+7)=25. You'd combine like terms, making it 2x+7=25. You'd then subtract 7 from both sides to get 18 and divide that by 2 to get 9. You have 9 treadmills. Finally, you'd plug 9 in for x in the equation y=x+7, to get 16 for stationary bikes.