1.5 minutes = 90 second
The probability density function is
[tex]f_X(x)=\begin{cases}\dfrac1{200-30}=\dfrac1{170}&\text{for }30\le x\le200\\\\0&\text{otherwise}\end{cases}[/tex]
so the probability of interest is
[tex]\mathbb P(X>90)=\displaystyle\int_{90}^\infty f_X(x)\,\mathrm dx[/tex]
[tex]\mathbb P(X>90)=\dfrac1{170}\displaystyle\int_{90}^{200}\mathrm dx[/tex]
[tex]\mathbb P(X>90)=\dfrac{200-90}{170}=\dfrac{11}{17}\approx0.6471[/tex]
