In 1960, what is largest population for country with upper middle income?
in 1960 refers to year = '1960'; the largest population refers to max(value where IndicatorName = 'Population, total'); country with upper middle income refers to incomegroup = 'Upper middle income'
SELECT MAX(T2.Value) FROM Country AS T1 INNER JOIN Indicators AS T2 ON T1.CountryCode = T2.CountryCode WHERE T1.IncomeGroup = 'Upper middle income' AND T2.Year = 1960 AND T2.IndicatorName = 'Population, total'