How can 4 resistors of 30ohms each be connected so you end up with a resistance of 18ohms?
Clearly to get 18 ohms, you need some of the resistors in parallel. 

So, what do you need in parallel with a 30 ohm resistor to get 18? Answer: 45 ohms.

Now, you have a simpler problem: how to make 45 ohms from three 30 ohm resistors. That should be obvious!

Some of the earlier answers have already "given you a fish," but "teaching you how to fish" is more useful in the long term IMO.