Birthday Cards

1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31

2
3
6
7
10
11
14
15
18
19
22
23
26
27
30
31
4
5
6
7
12
13
14
15
20
21
22
23
28
29
30
31
8
9
10
11
12
13
14
15
24
25
26
27
28
29
30
31
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31

With the five cards above, you can guess anyone's birthday!

Make the five separate cards by writing the numbers on a 3x5 card. Give the five cards to your friend, and ask them to hand you the cards that contain the number date of their birthday.

You secretly and quickly add up the first number on each of the cards your friend hands back to you. The sum is their birthday date -- in base two!

Remember that base two counts whether or not you need the place values 1, 2, 4, 8, 16, 32, etc to sum up the number. By picking the cards, your friend has indicated that the place value at the top of those cards will have a face value of one. The cards they did not pick are those that will have a face value of zero.

Practice with a partner until you can find your answer quickly. Then surprise all your friends!

More about base 2