Final answer:
To find the value of r1 that makes req, you need to know the individual resistances and their configurations (series or parallel), then use Ohm's Law and Kirchhoff's Laws to solve for r1.
Step-by-step explanation:
To find the value of r1 that makes req, we need to have more specific information about the problem. In physics, req usually refers to the equivalent resistance in a circuit. To calculate the equivalent resistance, you need to know the individual resistances and their configurations (series or parallel).
Once you have the necessary information, you can use Ohm's Law and Kirchhoff's Laws to solve for the unknown resistance, r1.
For example, if you have resistors in parallel, you can use the formula 1/req = 1/r1 + 1/r2 + 1/r3 + ..., where r1, r2, r3, etc., are the individual resistances. If you have resistors in series, you can simply add them up: req = r1 + r2 + r3 + ...