Answer
(a) $8.5$
(b) $2.4$
Work Step by Step
(a) We can find the f-number as
$f-number=\frac{f}{D}$
We plug in the known values to obtain:
$f-number=\frac{17}{2.0}=8.5$
(b) The f-number in dim light can be determined as
$f-number=\frac{f}{D}$
We plug in the known values to obtain:
$f-number=\frac{17}{7.0}=2.4$
