Final answer:
The error interval for the depth of the lake is 84 ≤ d < 85 m.
Step-by-step explanation:
The depth of the lake is given as 84 m, truncated to an integer. Since the depth is truncated, the actual depth could be anywhere within the range of 84 to 85 m.
To write this as an error interval, it can be expressed as a ≤ d < b, where a = 84 and b = 85.