(Calc I) WebWork says this is wrong? I've tried a few times and keep getting the same answer

[paper tapir]

anyone have an idea of what I'm doing wrong:
A street light is at the top of a 16 foot tall pole. A 6 foot tall woman walks away from the pole with a speed of 8 ft/sec along a straight path. How fast is the tip of her shadow moving when she is 30 feet from the base of the pole? I keep getting 24/5

[paper tapir]

<@&286206848099549185>

[worthy pasture]

Oh, yes, it's a small and subtle mistake.

Notice that s is the length of the shadow, so s' is how fast the shadow is growing.