For series combination
Req=R1+R2+...+Rn
For parallel combination of resistors
1Req=1R1+1R2+...+1Rn .
Therefore, 5 resistors of 2Ω in series will have Req=10Ω
4 resistors of 2Ω in series will have Req=8Ω
8 resistors of 2Ω in parallel will have Req=0.25Ω
4 resistors of 2Ω in parallel will have Req=0.5Ω
10 resistors of 2Ω in parallel will have Req=0.20Ω