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Step-by-step explanation:

27+36

=9×3+9×4

=9(3+4)

=9×7

=63

27+36=63

so 27+36=9(3+4)

Survey, observational study, observational study, survey

Step-by-step explanation:

In a observational study, the sample is only observed.  No variables are controlled.In a survey, information is obtained directly from the sample, using a questionnaire to measure opinions.In an experiment, variables are controlled by design to study a link of causation.

In the first one, the director wants to know what his singers do before a performance.  The director cannot observe each singer at the same time, so he should use a survey.

In the second one, the director wants to know who his best soprano is.  The best way to determine this is by simply observing.  Observational study.

In the third one, the directer wants to know how verbally disciplining a singer affects the group.  Again, the best way to determine this is by observing.  Observational study.

In the fourth one, the director wants to know what musical is the most popular.  This of course requires a survey.