Two ice skaters stand at rest in the center of an ice rink. When they push off against each other, the $45-\mathrm{kg}$ skater acquires a speed of $0.62 \mathrm{~m} / \mathrm{s}$. If the speed of the other skater is $0.89 \mathrm{~m} / \mathrm{s}$, what is that skater's mass?