DAY 51

Comment

Try numbers bigger than 15.
For example, 16 has 5 factors (1, 2, 4, 8, 16),
17 has 2 factors (1, 17),
18 has 6 factors (1, 2, 3, 6, 9, 18),
19 has ...

Answer

There are countless possible numbers: 21, 22, 26, 33, 34, 35, 38, 39, ...

Readers' methods

Anne W, hotmail:
Choose 2 prime numbers, eg 3 and 7,
and their product will give a number with 4 factors:
1, 3, 7, 21.

Suzanne R, Tain, Ross-shire IV20:
As any number has itself and 1 as factors,
in order for a number to only have 4 factors
the other two must normally be prime numbers,
ie have no other factors themselves.
So we can pick the next biggest prime number to 3 or 5,
5 and 7 respectively.
So the next biggest number with 4 factors is 3x7 = 21.
The next biggest breaks the prime number rule as it is 27,
however one of the factors, 3, is also the factor of the other factor, 9.

Michael T, Dundonald, Belfast BT16:
A number larger than 15 with 4 factors would be 21: 1, 3, 7, 21.

Celia W, Twickenham TW1:
For a number to have only four factors the factors other than itself and 1
have to be prime, otherwise they will factorise and make further factors
of the original number.
so 3x7=21 has factors 1, 21, 3 and 7;
3x11=33 has factors 1, 33, 3 and 11;
5x7=35 has factors 1, 35, 5 and 7;
5x41=205 has factors 1, 205, 5 and 41
and so on ...