1. Calculate the values of the brackets:
$\displaystyle \left(1-\frac12\right) \left(1-\frac13\right) \left(1-\frac14\right) \left(1-\frac15\right) ... \left(1-\frac1{50}\right) = \frac12 \cdot \frac23 \cdot \frac34 \cdot ... \cdot \frac{49}{50} =$
2. Cancel! You should come out with $\displaystyle \frac1{50}$
EDIT: ... too late - again!