These resistors are parallel These resistors are series

Combinations can be simplified so for example we can simplify this by using ohm's law for series on R3 and R4 to get what we will call x. Then combining our Value for that with R2 using ohms law we get the value y. Then we can find our new value for resistance by using ohms law on R1, R5 , y so mathematically: R3+R4=x ~>5+15=20=x