Add.Math problem, please help me><,tomorrow is the exam

[Mark Cheah]

question:solve sec(z/3)=4 for 0<z<5 radians
the answer is 3.96 and the range of z is still 0<z<5
i get the answer 3.96 and 14.9
but why the range of z is not 0<z<(5/3)?
This question is pass year paper may/june 2010 (10.iii)