There were 18 students assigned in a minibus for a field trip. Eight of these students were boys. On the day of the field trip, the number of girls and boys was the same since some of the girls were not able to join the trip. How many girls were not able to join the field trip?
8 boys + 8 girls = <<8+8=16>>16 students joined the field trip.
Thus, 18 - 16 = <<18-16=2>>2 girls were not able to join the field trip.
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