Write a number between 50 and 80 that has exactly 4 factors one of which is 2.

we want a number between 50 and 80 that has exactly 4 factors, where one is 2.

this means that our number X can be written as:

X = 2*a*b*c

where a, b and c are numbers that we do not know. We can start giving those numbers different values, and then changing them to get in the desired range:

X = 2*3*5*7  

i used the first 4 primes here,

X = 10*3*7 = 30*7 = 210

this is too big, let's reduce obviously we must reduce the 7, let's change the 7 for a 3

X = 2*3*5*3 = 10*9 = 90

We are still a little bit off, let's change the 5 for a 3.

X = 2*3*3*3 = 6*9 = 54

nice, we have found a number between 50 and 80 where the factors are 2 one time and 3 three times.