#### Answer

The diskette is:
$90\text{ mm}$ wide.

#### Work Step by Step

If we want to convert from inch to centimeters, we have to consider the following conversion rate:
$ 1\text{ in}=2.54\text{ cm}$
We multiply both sides by $3\frac{1}{2}$ in order to get the answer to the question:
$3\frac{1}{2} \text{ in} =3\frac{1}{2}\times 2.54\text{ cm}=8.89\text{ cm}$
After that, we convert from centimeters to millimeters:
$1 \text{ cm}=10\text{ mm}$
We multiply both sides by 8.89:
$8.89 \text{ ft}= 88.9\text{ mm}$
By rounding it to the nearest ten we can say that the diskette is:
$90\text{ mm}$ wide.