This would be labed as a Conditional probability
Let

Step 
Find 

Divide by
numerator and denominator


Step 
Find 

Divide by
numerator and denominator


Step 
Find 


Divide by
numerator and denominator


In this problem
The geometric sequence formula is equal to

For 
therefore
the answer is
the common ratio for the geometric sequence above is 
Answer:
B.
Mn = Mn-1 + 125 for n > 1 ; M1 = 3,875
Step-by-step explanation:
Since they tell us after 1970, the first data would be that of 1971 and also that if we started since 1970, we do not know the data of 1969, therefore answer B is correct.
Replacing:
Let M1 = 1971 then Mn-1, that is M0 = 1970, we know that the population of 1970 the population is 3750, because it would be 125 less than the subsequent year, and in 1975 there are 3875. Therefore:
in n = 1
M1 = M0 + 125
3875 = 3750 + 125
this gives an equality, thus fulfilling the equation.