For adults, primary school math looks easy, but it's hard.

I teach mathematics to a primary school student in the U.K. I struggle more with the time difference than with teaching. We are 8 hours ahead of London in Tokyo until this week. From next week, it will be 9 hours because of the end of daylight saving time.

Primary school math looks easy, but it’s hard for adults. Why don't you solve one of the questions I taught the student yesterday.

The question was from the Japanese junior high school entrance examinations.

Find the least number divided by 7 and 13, leaving the remainder 4 and 10, respectively.

Is it easy to answer ?

Divided by 7 leaves 4. 7-4=3
Hence, when 3 is added to the required number, it will be exactly divisible by 7

Divided by 13 leaves 10. 13-10=3
Hence, when 3 is added to the required number, it will be exactly divisible by 13.

LCM( 7, 13) is 91.

91 － 3 =88

Thus, 88 is required number.

How was it?

We need mental gymnastics.