Pick a two digit number. (23?)
Add the digits together. (2+3)=5
Subtract this sum from your original number. (23-5=18)

Stare at that position in the table:

00 01 02 03 04 05 06 07 08 09
10 11 12 13 14 15 16 17 18 19
20 21 22 23 24 25 26 27 28 29
30 31 32 33 34 35 36 07 08 09
40 41 42 43 44 45 46 07 08 09
50 51 52 53 54 55 56 57 08 09
60 61 62 63 64 65 66 67 68 69
70 71 72 73 74 75 76 77 78 79
80 81 82 83 84 85 86 87 88 89
90 91 92 93 94 95 96 97 98 99

Is it Red?
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If the number is not red, then you have some math drill coming your way:
A two digit number like 65 is really six times ten plus five,
and it is different from 56 which is five times ten plus six.

watch this trick:
six times ten plus five minus (six plus five)
six times ten plus five minus six minus five
six times ten minus six plus five minus five
six times ten minus six times one plus (zero)
six times (ten minus one)
six times (nine)
fifty-four !!!!
In symbols:
let t be the ten's digit and u be the unit's digit:
in 65, t=6 and u=5, 6*10+5, t*10 + u
in 93, t=9 and u=3, 9*10+3, t*10 + u
My two digit number is 10t + u.
The sum of the digits is t+u.
The difference of the original number and the sum is:

10t + u
-( t + u)
---------------
9 t

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