Q.2: Write five numbers which you cannot decide just by looking at their units digit (or units place) whether they are square numbers or not.
The five numbers can be: 521, 655, 124, 729, 1940, etc.
Basically, numbers ending with 0, 1, 4, 5, 6, or 9 at the unit's place can be square numbers.
Any natural number ending in 0, 1, 4, 5, 6 or 9 can be or cannot be a square-number. Five such numbers are:
56790, 3671, 2454, 76555, 69209
Property 2. If a number has 1 or 9 in the unit’s place, then its square ends in 1.
For example:
(1)2 = 1, (9)2 = 81, (11)2 = 121, (19)2 = 361, (21)2= 441.