Before going into the details of the problem, let us first write down the most important information required for solving this problem.
1 ft = 12 inches
The distance that was crawled by Jack = 4 ft
 = (4 * 12) inches
 = 48 inches
The distance crawled by Christine = 3/4 of the distance crawled by Jack
 = (3/4) * 48 inches
 = 3 * 12 inches
 = 36 inches
Then
The total distance crawled by the two children's = (48 + 36) inches
 = 84 inches
So Jack and Christine together crawled a distance of 84 inches.