The basic EV equation for a hero bet with fold equity is:
EV = Pr(fold)*Pot +(1-Pr(fold))*(eq*(Pot + 2*Bet)- Bet)
For your example where you bet 125 into a 100 pot with 25% equity and villain folds 40'% of the time, EV = 17.50
If you set EV = 0 , with some algebra (or my gwiz EV program), you will find the break-even fold equity to be 27.3%.
Note: If the bet is not a closing bet such as all-in or the hand is not assumed to be checked down, this will be a first-cut look at EV potential.