49
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21, 35, 49, 63, 77.
How about 80
A number is a multiple of 80 if the last 4 digits are a multiple of 80 The 1,000s digit is even and the last 3 digits are a multiple of 80 The 1,000s digit is odd and the last 3 digits are 40 times an odd number It also works if the number is a multiple of 5 and 16 at the same time because 80 = 2^4 x 5 = 16 x 5
There are 40 odd numbers between 20 and 100. Remember that an odd number is every second number. There are 80 numbers between 20 and 100, so there are 40 odd numbers and 40 even numbers.
7x9=63