Let |M| = students that failed in Maths, and |D| = students that failed in Digital.
$|M| = 875$
$|D| = 1050$
$|M \cap D| = 375$
To find: $| \overline{M}\cup \overline{D}| - | \overline{M} \cap \overline{D}|$
which is equivalent to finding: $| \overline{M \cap D}| - | \overline{M \cup D}|$
which is equivalent to finding: $2500- |M \cap D| - (2500-|M \cup D|)$
We know, $|M \cap D|$ = 375.
Now, $|M \cup D|$ = $875+1050-375=1550$
So, finally we have: $2500- |M \cap D| - (2500-|M \cup D|)$
=> $2500- 375 - (2500-1550)=1175$