.
If you need a translation to Math, here it is
n - 20 >= 15,
where "n" is "a number".
If you need a solution for "n", here it is
n >= 15 + 20,
or, equivalently,
n >= 35.
Solved.
===========================
Translation words to Math is not a tricky science.
See this source
https://www.palmbeachstate.edu/prepmathlw/Documents/translatingkeywords.pdf
and learn from there.
All the wisdom fits on one page.