b
\(P _{ i }= Nm v \hat{ i } \quad \overrightarrow{ P }_{ f }=- Nm v \hat{ i }\)

\(N\) is Number of balls strikes with wall

\(N =100\)

\(\Delta \overrightarrow{ P } =\overrightarrow{ P }_{ f }-\overrightarrow{ P }_{ i }=-2 Nmv \hat{ i }\)

\(=-200\,Nm v \hat{ i }\)

\(\overrightarrow{ F }_{\text {Total }} =\frac{\Delta \overrightarrow{ P }}{\Delta t }=-\frac{200 mvt }{ t }\)

\(|\overrightarrow{ F }| =\frac{200\,mv }{ t }\)