the answer is 12.
Get all the Prime number factor of each numbers and multiply all the common prime #.
144: 2*2*2*2*3*3 = 24*32=16*9
240: 2*2*2*2*3*5 = 24*3*5 = 16*15
612: 2*2*3*3*17 = 22*32*17 = 4*9*17=36*17
you can see that all three have 2*2*3 in common, which is equals to 4*3 = 12.
The greatest common divisor is 45.
The Greatest Common Divisor is 10
The Greatest Common Divisor is 4
The Greatest Common Divisor is 35
The Greatest Common Divisor is 28
The Greatest Common Divisor is 1
The Greatest Common Divisor is 19
The Greatest Common Divisor is 2
The Greatest Common Divisor is 4
The Greatest Common Divisor is 22
The Greatest Common Divisor is 9
The Greatest Common Divisor is 13