HI!
So the square root of 128 is 11.31.
You would know that it is more than 11 since 11^2 is 121 which is 7 less than 128.
So the answer would be between 11.3 and 11.4
also 12^2 is 144
121 and 144 are 23 points away and that divided by 10 is 23/10. 23/10 is a little more than 2. so everytime 121 is added by 2, go up by .1
so....
121+2 = 123
11 + .1 = 11.1
123+ 2 = 125
11.1 + .1 = 11.2
125+2=127
11.2+.1=11.3
127+2=129
11.3+.1=11.4
and you would know that square root of 128 is between 11.3 and 11.4
Hope this helps!