THREE DUMBBELLS

Each of these dumbbells is formed from dominoes in which the pips have been replaced by their corresponding digits. The outlines of the dominoes have been removed. In order to “lift” each dumbbell, you must use logic to draw the boundaries between the dominoes.
The dumbbell in the example is the lightest; it contains only 10 dominoes (0-0 through 3-3). The dumbbell in puzzle 1 contains 15 dominoes (0-0 through 4-4); in each successive puzzle, the range of digits increases by 1, and all of the dominoes forming each dumbbell are shown next to it. The heaviest dumbbell consists of a standard set of dominoes (0-0 through 6-6).
Can you prove your mental strength by locating all of the dominoes?

HINT