The first digit is 1, in the tens place. So the front end estimate is 10.
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Prime factorization.
I would go for 75 = 100*3/4 if the question involved multiplication or division. For addition or subtraction I would choose from 70 or 80 - depending on how the other term was being estimated.
You write down the multiples of those four numbers and find the LCM. EX. 1:1,2,3,4,5,6,7,8,9,10,11,12 2:1,2 3:1,3 4:1,2,3 So the LCM is 2.
In general, there is no simple method.
108 54,2 27,2,2 9,3,2,2 3,3,3,2,2 132 66,2 33,2,2 11,3,2,2 2 x 2 x 3 = 12, the GCF