175. Solving 1,5,6,7

private static IEnumerable<Expression> GetAllExpressions(IEnumerable<double> numbers)
{
    if (numbers.Count() == 1)
    {
        yield return Expression.Constant(numbers.First());
    }
    foreach (double number in numbers)
    {
        IEnumerable<Expression> expressionsWithoutNumber =
            GetAllExpressions(numbers.Except(new[] {number}));
        foreach (Expression expression in expressionsWithoutNumber)
        {
            yield return Expression.Add(Expression.Constant(number), expression);
            yield return Expression.Subtract(Expression.Constant(number), expression);
            yield return Expression.Multiply(Expression.Constant(number), expression);
            yield return Expression.Divide(Expression.Constant(number), expression);
        }
    }
}

private static IEnumerable<Expression> GetAllExpressions(IEnumerable<double> numbers)
{
    if (numbers.Count() == 1)
    {
        yield return Expression.Constant(numbers.FirstOrDefault());
    }
    foreach (double number in numbers)
    {
        var expressionsWithoutNumber =
            GetAllExpressions(numbers.Except(new[] {number}));
        foreach (Expression expression in expressionsWithoutNumber)
        {
            yield return Expression.Add(Expression.Constant(number), expression);
            yield return Expression.Subtract(Expression.Constant(number), expression);
            yield return Expression.Multiply(Expression.Constant(number), expression);
            Func<double> denominator =
                Expression.Lambda<Func<double>>(expression).Compile();
            if (denominator() != 0)
            {
                yield return Expression.Divide(Expression.Constant(number), expression);
            }
        }
    }
}

IEnumerable<Expression> expressions =
    GetAllExpressions(new double[] {1, 5, 6, 7});
foreach (Expression currentExpression in expressions)
{
    Expression<Func<double>> currentLambda =
        Expression.Lambda<Func<double>>(currentExpression);
    Func<double> currentDelegate = currentLambda.Compile();
    // We check when the expression represents 21
    if (currentDelegate() == 21) 
    {
        Console.WriteLine("21 = {0}", currentExpression);
    }
}
// 21 = (6 / (1 - (5 / 7)))