While trying to set up ign-math I encountered this error while trying to run the ruby example

My commands

cd ign-math/examples/
ruby ruby angle_example.rb

The error

Traceback (most recent call last):
    2: from angle_example.rb:23:in `<main>'
    1: from /usr/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in `require'
/usr/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in `require': cannot load such file -- ignition/math (LoadError)