Number Ninjas - private maths tuition

Math problem of the day

At McDonalds you can order Chicken McNuggets in boxes of 6, 9, and 20. What is the largest number such that you can not order any combination of the above to achieve exactly the number you want?


The answer is 43


For any desired number if it is divisible by 3 it can easily be made with 6 and 9 packs, except if the number is 3 itself. If you can't use all six packs then use one 9 pack and you can do the rest with six packs.

If the number is not divisible by 3 then use one 20 pack. If the remaining number is divisible by 3 then use the above method for the rest.

If the number still isn't divisible by 3 use a second 20 pack. The remainder must be divisible by 3, in which case use the 6 and 9 packs as above.

The largest impossible number would be such that you would have to subtract 20 twice to get a remainder divisible by 3. However, you can't make 3 itself with 6 and 9 packs. So the largest impossible number is 2*20+3=43.

Michael Shackleford, A.S.A.

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Number Ninjas

Private Maths Tuition for Grades 0 - 12

Maths literacy The aim of Number Ninjas is to restore and reinforce your child's confidence in mathematics.

Often maths marks begin to drop in senior primary school because basic skills and rules have not been grasped. Your child's age and standard related knowledge will be analysed and a remedial program will be individually designed to suit his or her needs.

I am a retired high school teacher with 25 years of experience. Students will be tutored individually, by myself, for an hour a week.

Number Ninjas aims to make children comfortable with numeracy, look forward to solving problems and develop a positive attitude to maths.

Number Ninjas is situated in the west of Johannesburg. For more information or to book an appointment please use the booking form on the left or call Lynda on 084 798 0210 , or send an email to

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If you live outside Johannesburg or are unable to make an appointment we provide tuition via email for only R199.00 per hour to anywhere in the world. Appologies, tuition in english only. We will send any relevant documents, tests and examples to your inbox.

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