Answer: 2b + 56.8 = 180.4
How I Got The Answer: To represent the number of the mass of a beaker, use the number b. Since the problem says, "Two beakers plus their contents have a mass of 180.4 (the contents being 56.8)," create the equation 2b + 56.8 = 180.4.
If you need to solve it, subtract 56.8 from both sides, canceling out 56.8 on the left side and turning 180.4 to 123.6. All you need to do now is divide 2 from each side, and you now have b = 61.8.