About a mathematical magic

Set the month of birth to x and the date of birth to y.

Formula:

(x×4+9)×25+y

=x×4×25+9×25 +y (split)

= 100x+225+y

Don't look at 225, as above: If the birthday on June 25th is regarded as 625 (a number), it can be written as 6× 100+25, just like 100x (month) +y (day).

In addition to 100x+y, there is another 225, and the result is 225, so the magician subtracts 225 in his mind, just like 850 in the question, and subtracting 225 equals 625 (June 25th). Birthdays are calculated like this.

As for the constellations, the magician has memorized the date of each constellation.

There is also the fastest answer from enthusiastic netizens, MINUS 2 is the month, and the last two digits MINUS 25 is the date. Take the previous "100x+y" and say it is negative (2× 100+25), which is 225.

Our teacher also played a similar game in class, (month ×4+6)×25+ date, and then said that he could guess our birthdays. Finally, I said that the teacher just subtracted 6×25= 150 from the answer.