In the context of your question, the equation \( a_n = a_1 \cdot r^{n-1} \) represents the growth of a population over time, where:
- \( a_n \) is the amount at time \( n \),
- \( a_1 \) is the initial amount (the amount at time \( n = 1 \)),
- \( r \) is the growth factor (in this case, the bacteria quadruple each day),
- \( n \) is the number of days.
From the information given, we know that after the first day (which is \( n = 2 \)), the population is 50. So:
- For the first day \( (n = 1) \), let's denote the initial amount of bacteria as \( a_1 \).
- \( r = 4 \) (because the bacteria quadruple each day).
The number of bacteria after the first day \( (n = 2) \) can be expressed as:
\[ a_2 = a_1 \cdot r^{2-1} = a_1 \cdot r \]
Since we know \( a_2 = 50 \):
\[ 50 = a_1 \cdot 4 \]
Now, we can solve for \( a_1 \):
\[ a_1 = \frac{50}{4} = 12.5 \]
Thus, the value of \( a_1 \) is 12.5.