
Sol: a)
{31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}.
It is a subset of 'N'.
b) The minimum sum is 2 (1 + 1) and the maximum is 12 (6 + 6).
∴ The set is: {2, 3, 4, . . ., 11, 12}.
It is also a subset of 'N'.
c) From the picture, the points that could be scored with each strike of the ball are: 0, 10, 20, 40, 50, 90, 100. Hence, if any one of the ball lands in zero score zone, the set is not a subset of 'N'. If none of the balls (in the six attempts) land in '0' score zone, the set is a subset of 'N'. 
Natural Numbers
The numbers which are used for counting are called natural numbers or counting numbers. The natural numbers set is an infinite set and is denoted by N.
Thus, N = {1, 2, 3, 4, 5, 6, 7, 8, . . .}.
Representation of natural numbers on number line: Draw a line. Mark points at equal intervals of length (say 1 unit), as shown below. Label these points as 1, 2, 3, 4, 5, . . . Then, these points will represent the natural numbers 1, 2, 3, 4, 5, . . . respectively on the number line. The arrowhead at one end of the number line indicates the continuation of these numbers indefinitely.
The number line shows the order of natural numbers. A number that appears to the left of a given number is less than (<) the given number. A number that appears to the right of a given number is greater than (>) the given number. So we can conclude that 1 < 2 < 3 < 4 < 5 . . .
Example: 
Are these subsets of natural numbers set (N) ?
a) No. of days in each month of a calendar year.
b) Sum of numbers when two dice are cast.
c) Points scored in wooden pinball game (Runner duck) say with 6 strikes. 
For solution refer to adjacent table. 