First you have to find the two perfect squares which are 64 and the square root of 64 is 8, and 81 which the square root would be 9. 66 is closer to 64 so that eliminates it down to 8.1, 8.2, 8.3, and 8.4. Half way between 64 and 81 is about 72.5 and between 64 and 72.5  66 is closer to 64 so i estimate the square root of 66 is 8.12 and so on.