Multiply all the digits in a number
Repeat until you have a single digit left
As explained by Numberphile:
Numberphile "What's special about 277777788888899?"
I've been posting relatively hard challenges recently, so here goes an easy one.
Given an array \$A\$ and a number \$n\$, calculate the sum of all numbers of \$A\$ multiplied by \$n\$, except the ...
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