As one of the denominations is rupee 1 , so we can reduce the computation by checking the equality for N - ( A * 1 ) = N - A
for ( int b = 0 ; b <= B && b * 5 <= ( N ) ; b ++ ) for ( int c = 0 ; c <= C && b * 5 + c * 10 <= ( N ) ; c ++ ) for ( int d = 0 ; d <= D && b * 5 + c * 10 + d * 20 <= ( N ) ; d ++ ) if ( ( b * 5 ) + ( c * 10 ) + ( d * 20 ) >= ( N - A ) )