Since you only have 20 and 50 ohm resistors, you must place resistors in parallel to achieve the desired resistance.
When resistors are parallel the equivalent resistance is given by:
R1 // R2 = 1 / (1/R1 - 1/R2) (* // = parallel)
There are more than one way to obtain the 45 total ohm resistance,
For example a (50 // 50) = 25 ohm in series with a 20 ohm to that and you get 45 ohm.
Part B, is solved the same way.
For example use a (20//20) = 10 ohm in series with another 20 ohm resistor for a total of 35 ohms.