🏈 Two Lines Joined at 50

A football field is two number lines back to back. Put it on one line and every spot has one number.

Two number lines joined at 50

A football field counts up from each goal line to 50 in the middle, so most numbers name two places: your own 30 and their 30 are 40 yards apart. Put the field on ONE number line, 0 at your goal and 100 at theirs.

On the single line, where is "their 30"?