Answer: 29 mm or 2.9 cm
Explanation:
Given : The length of grasshopper = 99 mm
The length of cockroach = 7 cm
We know that 1 cm = 10 mm
Then , the length of cockroach = 70 mm
Then , the difference in length between the insects is given by :-

Therefore , the difference in length between the insects = 29 mm or 2.9 cm