Let \( x \) represent the number of minutes Jamyra studied last week. According to the problem, this week she studied a total of 100 minutes, which is described by the equation:
\[ 100 = 4x + 8 \]
To solve for \( x \), we first subtract 8 from both sides:
\[ 100 - 8 = 4x \]
This simplifies to:
\[ 92 = 4x \]
Next, we divide both sides by 4:
\[ x = \frac{92}{4} = 23 \]
Thus, the number of minutes Jamyra spent studying last week is \( \boxed{23} \).
To verify, we can substitute \( x = 23 \) back into the context of the problem. Calculating four times last week's study:
\[ 4 \times 23 = 92 \]
Adding 8 to check the total for this week:
\[ 92 + 8 = 100 \]
This confirms that her total for this week is indeed 100 minutes. Therefore, our answer is correct and Jamyra spent \( \boxed{23} \) minutes studying last week.