We assume "1/3 of Leah’s marbles is equal to 4/5 of Dan’s marbles.".

Let 'x' represent the number of Leah's marbles and 'y' represent the number of Dan's marbles.

x + y = 28

(1/3)x = (4/5)y

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click here to see the system of equations solved for x and y

........

x = 19.8

y = 8.2

Since the number of integers can only be integer and the solutions of the system of equations are not integers the problem have no solutions.