Please explain everything! show work! please...

Consider the following loop where $n=2^k$ for $k0$ :

sum larr$0$;

for $(ilarr1$;$in$;ilarr$2i)do$

for $(jlarri$;$jn$;$j++)do$

sum larr sum $+1$

$($a$)$ Count the number of times that sum larr sum $+1$ is executed.

$($b$)$ What is the efficiency class of the loop?