don steward
mathematics teaching 10 ~ 16

Tuesday, 8 February 2011

digits 1 to 9 divisions

without a calculator, divide 12,345,678 by 9
now divide 123,456,789 by 9 (why was this easy?)

use the digits 1 to 9 once only

in the sum:

students who have some familiarity with divisibility tests (or have just done work on this) will appreciate that the single digit cannot be 3, 6 or 9 because the result isn't divisible by 3
or 5 because the result doesn't end with 5 or 0
or 1, which works but this is a trivial case

all other options for the single digit could work...

an intention is that students divide 98765432 by each of the remaining four digits to see if they work...
2 nearly does
4 nearly does
7 doesn't work without a remainder
and 8 does work

so there is only one solution (discounting the trivial case)

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