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9 Mar 2021追記 各カテゴリ3問以上できて見た目も整ったので今後は中等部の記事拡充を行います。ここの更新はゆっくりになります。
May 2021 高校内容を扱う高等学院ができました。Ninja Math Dojoは発展問題を中心にゆるゆると追加していきます。
1 Basic algebra
1-1 Sum/difference-calc (和差・差集め算)
1-2 Crane-tortoise-calc (鶴亀算) 
1-3 Deficiency/excess-calc (過不足算)
1-4 Reasoning (推理算)
1-5 Arithmetic operators (演算記号)
1-6 Newton-calc (ニュートン算)

2 Rate
2-1 Ratio-calc (比・倍数算)
2-3 Age-calc (年齢算)
2-4 Work-calc (仕事算)
2-5 Gain-and-loss-calc (売買・損益算) 
2-6 Concentration-calc (濃度算)

3 Speed
3-1 Speed-time-distance (旅人・通過算・速さの問題)
3-2 River-flow-calc (流水算)
3-3 Clock-calc (時計算)

4 Pattern
4-1 Tree-planting-calc (植木算)
4-2 Sequence/pattern/square pattern (数列・方陣算)
4-3 Calendar-calc (暦算)
4-4 Numeral systems (N進法)

5 Probability/permutation/combination (場合の数・確率)

6 Geometry
6-1 Plane figures (平面図形)
6-2 Space figures (立体図形)

7 Misc (その他)

8 Advanced (発展問題)


Misc 5


Question (3:17~)
Ichiro and Jiro went to pick mushrooms in the mountains. At first, the ratio of Ichiro’s mushrooms to Jiro’s was seven to three. However, Ichiro lost seven mushrooms on the way home, while Jiro newly found six and took them. After arriving home, the ratio of Ichiro's mushrooms to Jiro's was four to three. Find the number of mushrooms that Ichiro took at the beginning.

Jun’s answer (3:50~)
Ichiro, are you such an idiot that you dropped seven mushrooms without noticing it? I suspect that the six mushrooms Jiro took could be yours!? If there is an answer to this question, although it is not good to doubt people around you, perhaps you should avoid Jiro.



Speed-time-distance 7

Aoi and Beniko ride bicycles on a 730m circular course without stopping pedalling. Aoi’s travelling distance with 3 pedallings of a bike is the same as Beniko’s travelling distance with 4 pedallings. Beniko can pedal 11 times within the time in which Aoi pedals 10 times.
(1) Find the speed ratio of Aoi to Beniko in the simplest form.
(2) How many meters does Beniko travel while Aoi makes 4 rounds of the course?
(3) Suppose that single pedalling of Aoi’s bike makes a 5m travel. Aoi and Beniko start from the same point and travel in opposite directions. How many times does Beniko pedal until they meet?

(桜美林中 https://www.obirin.ed.jp/



Advanced 7

A car travelled on an outbound lane of a highway for two days. Suppose all cars travelling inbound on this highway travel at 90km/h at a uniform distance from each other and the number of inbound cars per km varied between the 1st and 2nd day. Answer the following questions.
(1) On the 1st day, the outbound car travelled at 72km/h and it passed each other with 216 inbound cars in 60 seconds.
(1-1) On the 1st day, how many inbound cars passed in 60sec in front of a traffic camera over the highway?
(1-2) How many cars per km were travelling inbound on the 1st day?

(2) On the 2nd day, there were 75 cars travelling inbound per km. In a certain 60 seconds duration, the outbound car travelled at 72km/h for some seconds and 90km/h for the rest. The outbound car passed each other with 216 inbound cars in this duration. How many seconds did the car travel at 72km/h?

(早稲田中 https://www.waseda-h.ed.jp/