Determine the unknown time period. The population of Pleasantville was 8,550 people and increased by 2.5% each year to 15,851 people. Determine the number of years between the population records.

For the answer to the question above. Are you asking on how to add y + 1.2y + 1.2z ?The answer is just
2.2y + 1.2z
you just have to add the common terms since you have both (1)y and 1.2 y same as y(1+1.2) = 2.2y
since there's no z term, it's just 1.2z so you get
2.2y + 1.2z
This is just a simple problem of addition on algebra.