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The lower class limit is defined as the smaller number in the interval.
Thus, the lower class limit of 15-18 is 15.
Hence, option 1 is the correct option.
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If the upper and the lower limits of a class interval are 16 and 10, the class-mark is :
6
3
13
none of these
If the lower and the upper limits of a class interval are 7.5 and 12.5, the class-mark is :
10
2.5
7.5
12.5