For 1b, first short out the 10V source first, then draw your loops and find I due to the 15V source; next, leave the 10V source in the diagram, but this time short out the 15V source, then draw your loops and find I due to the 10 V source. Your final result should be I = I due to 15V source + I due to 10 V source.
For 1c, draw 3 loops of current, I1, I2, and I3, in cw or ccw direction (they should be in the same direction to avoid confusion). Use Kirchoff's Laws to lay out an equation for each loop. Then solve the 3 by 3 linear equation for the loop currents. For I, it should be the total amount of current flowing through that particular part, which is I1-I2 or I2-I1 depending on your directions. From that point on, you should be able to get I.