Which number should replace the question mark?
Check your answer:-
This is following this pattern
A*B – (B-A) – 1
for first line 4*6 – (6-4) -1 => 24 – 2 -1 => 21
you can see the same for all other lines, thus for the last one
4*4 – (4-4) – 1 => 15
