Let the ages of the five siblings be represented as follows:
- Oldest sibling: \( a + 8 \) (Key is the youngest, so the oldest is 8 years older)
- Second oldest: \( a + 6 \)
- Middle sibling: \( a + 4 \)
- Second youngest: \( a + 2 \)
- Key (youngest): \( a \)
We know all siblings together are 70 years old, so we can set up the equation:
\[ (a + 8) + (a + 6) + (a + 4) + (a + 2) + a = 70 \]
Combining like terms:
\[ 5a + 20 = 70 \]
To isolate \( a \):
\[ 5a = 70 - 20 \] \[ 5a = 50 \] \[ a = 10 \]
Thus, the equation \( 5a + 20 = 70 \) can be used to determine the age \( a \) of Key.