The obvious way is to break them up in two groups, and weigh those against each other. Choose the heavier group and repeat. If there is an odd number, add one ball from a previous weigh. Each side will contain the following in consecutive weighs : 50 , 25, 13, 7, 4, 2, 1. so 7 weighs in total.

A less obvious way would be to divide each group in to 3 groups. Weigh two against each other. If one is heavier choose that group and continue. If both sides are equal, then the heavier ball is in the group that wasn't weighed.

The largest group in each weigh in the WCS - 34, 12, 4, 2, 1 - so 5 weighs.

5