It can be solved by Exponential Distribution
f(x) = λe^{-λx} if x> 0
0 if x< 0
The cumulative distributive function F(a) of an exponential random variable is given by
F(a) = P(x≤a) = ∫_{0}^{a } λe^{-λx} dx = 1- e^{-λ*a}
a) P(x>10) = 1- P(x<10)
= 1- (1- F(10))
= 1 - (1- e^{-λ*10}) = e^{-1 }= 0.368
b) P(10<X<20) = F(20) - F(10)
= (1-e^{-λ20}) - (1-e^{-λ10}) = e^{-1 }- e^{-2 = } .233