Calculate the force in the biceps when the arm is flexed at 90 degrees during a slow 10 lb. biceps curl?

Question

Assume weight of lower arm, location of biceps attachment, etc.)

Answer 1

Assume weight of lower arm ... is what? Ignored? OK.
Call the distance from the elbow joint to:
the weight L1
the location of biceps attachment L2
Then you have a simple lever. F1 * L1 = F2 * L2
If we're not ignoring the lower arm weight W, add W/2 to the F2 which you got from the above equation.