arithmetic
posted by AD .
One chicken lays an egg every 2 days, another every 3 days and a third chicken lays an egg every 5 days. They all lay an egg today. How many days in the next 101 days will EXACTLY 2 chickens be laying an egg?

It sounds as if you need to find a good calendar with big blocks and three pens of different colors.
Dive in... 
First, you have to find the least common multiple of the number of days between eggs (2,3,4) to get 30. Then, you make a list of numbers 130:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30