If this is supposed to be a "what's the worst case" sort of question --

Drop first at the 50th floor. If it shatters, work you way up from the first floor. If it does not, drop from the 75th floor.

If it shatters, work up from 50th floor. It it does not, drop from the 88th floor.

If it shatters, work up from the 75th floor. It it does not, drop from the 94th floor, etc.

One test.

If they shatter, they will shatter at the bottom floor.

=)D