Given:
x501=12⇒x=1250
y501=18⇒y=1850
So,
x+y=1250+1850
⇒25x+y=251250+251850
⇒25x+y=25(123)16122+25(182)25
⇒25x+y=25(1728)16122+25(325−1)25
⇒25x+y=25(1725+3)16122+25(325−1)25
So, remainder is
25316122−1=25(33)5×3×144−1
=25(25+2)5×3×144−1
=25(25+2)5×3×(150−6)−1
=2525×3×(−6)−1
=25(25+7)×3×(−6)−1
=25(21)×(−6)−1
=25(25−4)×(−6)−1
=25(−4)×(−6)−1
=24−1=23
Hence, the remainder is 23