The figure (Figure 1) shows voltage and current graphs for a resistor.

I found part A , emf frequency = 25 Hz

Part B

What is the value of the resistance R?

Part C

Draw the resistor’s voltage and current phasors at t=15ms.

R = V/i = 10/0.5 = 20Ω

V(t) = 10cos50πt = 10cos9000t where 9000t is in degrees because 50π rads/sec * 180°/π radians =9000

i(t) = 0.5cos50πt = 0.5cos9000t where 9000t is in degrees

At 15ms V = -7.07 and i = -0.3536A and they are exactly in phase with each other. 9000*15e-3 = 135°