A new fizzbuzz: generating sums for kids

With my oldest quizzing me for one sum after another for him to solve. I thought about writing a little piece of R code that would generate these for me. The code for doing so was less obvious then I imagined it to be.

Goal:

Generate all possible sums and subtractions for kids of the form

10 - 4 = ?
4 + 3 = ?

Constraints:

- Only numbers between 0-10
- Result should be >= 0

Solution:

generate_sums <- function(maxresult=10) {
  sums <- c()
  for (i in 0:(maxresult-1)) {
    for (j in 0:(maxresult-i)) {
      sums <- c(sums, paste0(i, ' + ', j, ' = ?'))
    }
  }
  sample(sums[!duplicated(sums)])
}
generate_subtractions <- function(maxresult=10) {
  substractions <- c()
  for (i in 0:maxresult) {
    for (j in i:maxresult) {
      substractions <- c(substractions, paste0(j, ' - ', i, ' = ?'))
    }
  }
  sample(substractions[!duplicated(substractions)])
}

I still believe that a more elegant approach should be possible but at least it works.