I'll just comment on your answer to the first problem. Your Thevenin resistance and open circuit voltage calculations are correct. The maximum power transfer occurs when the load resistance equals the Thevenin resistance, which is 12.5 ohms. The current in such a situation is then
Hence, the power dissipated in the load resistor alone is
Finally, placing a 7 ohm resistor from b to a would result in a voltage divider just as you have there.
In short, your answer for (i) is completely correct, though you might want to check your significant figures for the voltage divider at the end.
If your second two problems are equally careful, then you're probably fine.