Let x represent the number of erasers Gabriel collected.
Celine collected 2*x erasers.
Julian collected 2*x*2=4*x erasers.
In total, Gabriel, Celine and Julian collected x+2*x+4*x=7*x erasers.
7*x=35
x=<<5=5>>5
Celine collected 5*2=<<5*2=10>>10 erasers.
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