Ace engineering academy OS IPC question 10

Question

Can someone please explain the solution to this problem? 

 

Answer 1

$A=1,B=0$

$P_{1}$	$P_{2}$
$P(B)$	$P(A)$
$R_{1}$	$R_{1}$
$R_{2}$	$V(B)$
$V(A)$	$P(A)$
$P(B)$	$R_{2}$
$R_{3}$	$R_{3}$
$R_{4}$	$V(B)$
$V(A)$	$P(A)$
	$R_{4}$

 

This satisfies all the given conditions. So, $2$ binary semaphores are required.

Correct me if I'm wrong