Calculating feedback resistors for op-amp based voltage regulator
First off, a 741 will never work. Between the feedback network and the load, the op amp needs to provide roughly 50 mA, which is way beyond a 741's capabilities.

To do this the easy way, replace R1 with a 9k resistor. When the op amp is running properly, the voltage across this resistor will be 9 volts, for a total current of 1 mA. When the output is at 20 volts, the voltage across pot and R2 will be 11 volts. Since the current through these components must also be 1 mA, the total resistance must be 11k. Likewise, at 25 volts the resistance must be 16k. The difference is 5k, so that's the pot value, and the remainder is 11k, so that's R2. Any other set of values may be found simply by multiplying each of the three resistances by the same amount.

To address the current problem, simply add and NPN transistor like so







simulate this circuit &ndash; Schematic created using CircuitLab

For what you're doing, almost any NPN will work, such as a 2N3904, although it will get pretty warm. If you decide to drive a lower resistance, you'll need to calculate the power dissipation of the NPN. And be aware that if you accidentally short the load resistor, you will instantly kill the transistor.