Why is resistance less in parallel combination of resistors ?

(a) Why does resistance increase in a series combination of resistors ? (b) Why does resistance decrease in a parallel combination of resistors ?

In parallel combination of resistances :

Why is resistance more in series combination of resistors ?

In parallel combination of resistance-

Find the effective resistance of the parallel combination of three resistors of resistances 20 Omega , 40 Omega and 50 Omega .

What is meant by Parallel combination of resistors.