A student in the laboratory measures thickness of a wire using screw guage, the reading are 1.22 mm, 1.23 mm, 1.19 mm and 1.20 mm, the percentage error is $\frac{x}{121}\%$  , the value of  x is ______

Mean thickness 
 $t_{mean}=\frac{t_1+t_2+t_3+t_4}{4}$ $=\frac{1.22+1.23+1.19+1.20}{4}$ ;   $t_{mean}=1.21$
Error in measurement are 
$e=|t_m-t_i|$ ;   $e_1=|1.21-1.22|=0.01$
Similarily, 
 $e_2=0.02,e_3=0.02,e_4=0.01$
Mean error 
 $\overline{e}=\frac{e_1+e_2+e_3+e_4}{e_1}=0.015$
Percentage error
$=\frac{e_{mean}}{t_{mean}}\times 100 =\frac{0.015}{1.21}\times 100 =\frac{150}{121}\%$   ;  $x=150$