let

j = the number of seashells Jackson collected

p = the number of seashells Petra collected

t = the number of seashells Tyrone collected

Jackson, Petra and Tyrone went to the beach and collected seashells for a total of 94 shells

j + p + t = 94

Petra and Tyrone each collected 13 fewer than twice the amount of Jackson

p = 2j - 13

t = 2j - 13

by solving the system of equations

j + p + t = 94

p = 2j - 13

t = 2j - 13

we find

j = 24 shells

p = 35 shells

t = 35 shells

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Jackson collected 24 seashells.

Petra and Tyrone collected 35 seashells each.