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

Explanation

(a) Connecting a number of resistors in series is equivalent to increasing the effective length (l) of the conductor. Recall `R prop l`.
(b) Connecting a number of resistors in parallel is equivalent to increasing the effective cross-sectional area (A) of the conductor Recall `R prop 1//A`.