Answer:
18. Sqrt40 goes between 6 and 7.
19. Sqrt83 ~= 9
Step-by-step explanation:
For both of these problems, it's really useful to be very familiar with the perfect squares on the times table. 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225...
On #18, we're looking at the sqrt40. This will be between sqrt36 and sqrt49, because 40 is between 36 and 49. But sqrt36 is 6 and sqrt49 is 7. So sqrt40 is between 6 and 7.
For #19, the sqrt83 is far closer to sqrt81 than it is to sqrt100. Sqrt81 = 9. So sqrt83 is very close to 9, it would round to 9 and not 10.