Oracle's Riddle

You're asking the Oracle to foretell when you would meet your soul mate.

Oracle : You'll have a 52% chance to meet your soul mate tomorrow. Whether you'll see her today will affect whether you'll see her tomorrow.

You : Then what is my chance of meeting her today?

Oracle : I shall not speak Heaven's truth. All I can tell you is that your chance of seeing her tomorrow will be doubled if you see her today, and the chance of not seeing her tomorrow will be tripled if you don't see her today.

What is the probability (in percentage) of seeing your soul mate today?


The answer is 30.

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1 solution

By choice, we have four scenarios of whether we'll meet the soul mate today and whether we'll meet tomorrow, as shown above. Note that the red legs (no meeting today choice) have y + ( 1 y ) = 1 = 100 % y + (1-y) = 1 =100\% .

Let x x be the chance of meeting the soul mate today and y y be the chance of meeting her tomorrow if we don't see her today. Thereby, the chance will be doubled to 2 y 2y if we see her today.

Likewise, if we don't see her today, the chance of not seeing her tomorrow will be 1 y 1-y , complemented to y y . This chance is three times that of 1 2 y 1-2y , another complement to 2 y 2y .

Thus, 1 y = 3 ( 1 2 y ) = 3 6 y 1-y = 3(1-2y) = 3-6y . y = 0.40 = 40 % y = 0.40 = 40\% .

Then 52 % = x ( 2 × 40 % ) + ( 1 x ) ( 40 % ) = ( 40 % ) x + ( 40 % ) 52\% = x(2\times 40\%) + (1-x)(40\%) = (40\%)x +(40\%) .

12 % = ( 40 % ) x 12\% = (40\%)x

x = 30 % x = \boxed{30\%} .

Checking for tomorrow's no meeting chance = ( 30 % ) ( 100 % 80 % ) + ( 70 % ) ( 100 % 40 % ) = 6 % + 42 % = 48 % (30\%)(100\%-80\%) + (70\%)(100\%-40\%) = 6\% + 42\% = 48\% , complementing the original 52 % 52\% in the question.

Thank you for your solution, but, honestly, I don't get it. Why 1 - y is divided by 3 when the problem states that the chance of not seeing (1 - y) will be tripled? In my mind it should be 3 * (1 - y).

Antonina Nesterenko - 2 years, 12 months ago

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The chance of not seeing is tripled if you don't see today. Please relate to my diagram: relatively you see her today, the chance of not seeing is thus divided by 3.

Worranat Pakornrat - 2 years, 11 months ago

The problem implies that pr ( meet her not tmr | meet her not today) = 3pr ( meet her not tmr | met her today)

Yuan Zhi Lee - 1 year ago

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