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An arithmetic Le Monde mathematical puzzle (that first did not seem to involve R programming because of the large number of digits in the quantity involved):

An integer x with less than 100 digits is such that adding the digit 1 on both sides of x produces the integer 99x. What are the last nine digits of x? And what are the possible numbers of digits of x?

The integer x satisfies the identity

where ω is the number of digits of x. This amounts to

10….01 = 89 x,

where there are ω zeros. Working with long integers in R could bring an immediate solution, but I went for a pedestrian version, handling each digit at a time and starting from the final one which is necessarily 9:

#multiply by 9
rap=0;row=NULL
for (i in length(x):1){
prud=rap+x[i]*9
row=c(prud%%10,row)
rap=prud%/%10}
row=c(rap,row)
#multiply by 80
rep=raw=0
for (i in length(x):1){
prud=rep+x[i]*8
raw=c(prud%%10,raw)
rep=prud%/%10}
#find next digit
y=(row[1]+raw[1]+(length(x)>1))%%10

returning

7 9 7 7 5 2 8 0 9

as the (only) last digits of x. The same code can be exploited to check that the complete multiplication produces a number of the form 10….01, hence to deduce that the length of x is either 21 or 65, with solutions