How many 2-digit numbers are there which has only 3 factors (including 1)?

A factor of a number is another number that divides it evenly with no remainder.   For example, the factors of 8 are 1, 2, 4, and 8. The sum of the factors of a number is the total of all its factors. There are several formulas to calculate the sum of the factors of a number. For example, the sum of factors of a number \(N\) where \(N=am\times bn\) is: \((a0+a1+......+am)\times (b0+b1+............+bn)\)  For example, the sum of factors of 100 is 217. \(100=22\times 52\) \((20+21+22)\times (50+51+52)\) \(\Rightarrow (1+2+4)\times (1+5+25)\) \(\Rightarrow 7\times 31\) \(\Rightarrow... Show more

A factor of a number is another number that divides it evenly with no remainder.  

For example, the factors of 8 are 1, 2, 4, and 8. The sum of the factors of a number is the total of all its factors.

There are several formulas to calculate the sum of the factors of a number.

For example, the sum of factors of a number \(N\) where \(N=am\times bn\) is: \((a0+a1+......+am)\times (b0+b1+............+bn)\) 
For example, the sum of factors of 100 is 217. \(100=22\times 52\) \((20+21+22)\times (50+51+52)\) \(\Rightarrow (1+2+4)\times (1+5+25)\) \(\Rightarrow 7\times 31\) \(\Rightarrow 217\) 
 
The sum of factors can be used in a number of applications, such as finding the number of divisors of a number or finding the probability of an event.

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