SELECT sum(T2.hours) FROM assignedto AS T1 JOIN projects AS T2 ON T1.project  =  T2.code JOIN scientists AS T3 ON T1.scientist  =  T3.SSN WHERE T3.name  =  'Michael Rogers' OR T3.name  =  'Carol Smith'
What is the sum of hours for projects that scientists with the name Michael Rogers or Carol Smith are assigned to?
[Schema (values) (types)]: | scientist_1 |  Scientists : ssn (text) , name (number) | Projects : code (text) , name (number) , hours (text) | AssignedTo : scientist (text) , project (number);