Simplify: 9(a + b) + 4(3a + 2b)
a) 13a 13b
b) 21a + 17b
c) 21a + 2b
d) 38ab

Identify all the cases where non-probability sampling is used when attempting to study the characteristics of a large city. studies on every subject in the population divide a population in groups and obtain a sample studies with a limited amount of time demonstrate existing traits in a population studies with the nth person selected