If you're talking about a discrete distribution, then
[tex]P(X=x)=\begin{cases}\frac1{61}&\text{for }x\in\{10,11,12,\ldots,70\}\\0&\text{otherwise}\end{cases}[/tex]
(1/61 because there are 61 numbers in the range of integers from 10 to 70) so that
[tex]P(X<25)=\displaystyle\sum_{x=10}^{24}P(X=x)=\boxed{\dfrac{15}{61}}[/tex]
If the distribution is continuous, we would have the same density function, but x can be any real number in the interval [10, 70]. So we have
[tex]P(X<25)=\displaystyle\int_{-\infty}^{25}P(X=x)\,\mathrm dx=\frac1{60}\int_{10}^{25}\mathrm dx=\dfrac{15}{60}=\boxed{\dfrac14}[/tex]
