this must be basic maths but i am missing something & i am stuck.

a . shorted 145 qty at 324.45 –

b. shorted another 4 qty at 325.10

the position book reflects 149 qty avg sell price 324.47

so, how is avg sell price as 324.47 ??

my maths is avg (324.45+325.1)/2 = 324.78

Whats the maths involve here? must be basic, but i admit, i don’t know… Please help. God bless you really.