# Kirchhoff's laws circuit, How to calculate unknown resistor?

First time here! I got a question on a test that I cant get my head around.

• How do I calculate value of R2 ?
• Is it possible ?

• There's a built-in schematic editor on this site, if you want to make your schematic look nicer. – Hearth Mar 12 '19 at 15:26
• Yes it's possible. You have three equations and constraints. HINT: Solve for the power dissipation of R2 in terms of I. Another hint: R2 is between 50 and 100 ohms, and the loop current is less than 250mA – BB ON Mar 12 '19 at 15:35
• Thanks! I think i got it now! – Simon Watti Mar 12 '19 at 16:03

$$I = \frac{10 V}{R_2 + 15 \Omega}$$
$$1W = P = I^2R_2 = \biggl(\frac{10 V}{R_2 + 15 \Omega}\biggl)^2 \times\ R_2$$