3. Given two geometric progressions $a_{n}$ and $b_{n}$. It is known that $a_{0}=4$ and the sum of the denominators of these two progressions is 5. Find what $b_{0}$ can be, if it is known that $a_{0}, b_{0}, a_{1}, b_{1}$ are consecutive terms of some arithmetic progression (in that order)
Answer: $-2 ; 8 \| 8 ; -2$

## Examples of answer notation:

17

$1 / 7$

1.7

$1 ; 7$

1, $7 ; 17$

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