Three days before the day after tomorrow is Monday. What day is today?
List out days of the week:
Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday
Three days before the day after tomorrow means we start 2 days from now, and go back three days to get to Monday.
Which means it's a...
Olga wrote all the natural numbers from 1 to k. Including 1 and k. How many numbers did she write?
The formula for the number of numbers including A to B is:
B - A + 1
With A = 1 and B = k, we have:
k - 1 + 1
k
C = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19}
Notice, we do not include 20 since it said less than, and not less than or equal to.