You can do
[5, 10].min
or
[4, 7].max
They come from the Enumerable module, so anything that includes Enumerable will have those methods available.
v2.4 introduces own Array#min and Array#max, which are way faster than Enumerable’s methods because they skip calling #each.
@nicholasklick mentions another option, Enumerable#minmax, but this time returning an array of [min, max].
[4, 5, 7, 10].minmax
=> [4, 10]