p(X>20 millsec) = 1 – p(X<= 20 millsec) ----- (1)
PDF = lamba. e^(-lamba.x)
CDF = p(X<= 20 millsec) = integral x from -infinity to 20 (PDF).dx
= 1 – e^(-lamba*20) ( given lamba = 1/20)
= 1 – e^-1 -------(2)
sub 2 in 1.
p(X>20 millsec) = 1 – (1 – e^-1) = e^-1 = 1/e = 0.36787944117 ~ 0.368 : option(D)