All multiples of 6 are even numbers.
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'5' & '15' are odd numbers less than 20 , and are also multiples of '5'.
4 x 5
The squares -- nine of them.
There are 9 integers less than 100 that have an odd number of factors.Every factor of a number has a pair, eg 2 & 3 are a factor pair of 6; and so it would be expected that every number has an even number of factors.However, if the factor pair of a number are the same number (eg 6 & 6 are a factor pair of 36), then there will be an odd number of factors.When there is a repeated factor like this, the number is a perfect square.Thus only perfect squares have an odd number of factors.Less than 100 there are 9 perfect squares (1, 4, 9, 16, 25, 36, 49, 64 & 81) which have an odd number of factors.100 itself is a perfect square and also has an odd number of factors, but the question asked for those numbers less than 100 with an odd number of factors.
1st odd number . . . 1 (1 less than double 1)2nd odd number . . . 3 (1 less than double 2)3rd odd number . . . 5 (1 less than double 3)4th odd number . . . 7 (1 less than double 4)5th odd number . . . 9 (1 less than double 5)Are you seeing a pattern yet ?6th odd number . . . 11 (1 less than double 6)7th odd number . . . 13 (1 less than double 7)...Qth odd number . . . . (1 less than double Q)