How to understand this Standard Deviation question?

Here’s the question:

The lengths of a sample of tiger canines were measured. 68 % of the lengths fell within a range between 15 mm and 45 mm. The mean was 30 mm. What is the standard deviation of this sample?

A. 5 mm
B. 15 mm
C. 7.5 mm
D. 30 mm

I just don’t really understand how to calculate the SD if I don’t have the number of values in the sample. Thanks!