How much is the total urban population of middle income countries in 1960?
IncomeGroup = 'Middle income'; Year = 1960; urban population refers to IndicatorName;
SELECT SUM(T2.Value) FROM Country AS T1 INNER JOIN Indicators AS T2 ON T1.CountryCode = T2.CountryCode WHERE T1.IncomeGroup LIKE '%middle income' AND T2.Year = 1960 AND T2.IndicatorName = 'Urban population'