How do i add a random score made to another score in a game software? How do i make THAT control?

I'm looking to make a trivia team game however i'm still learning about how to make the controls. These in particular: in the right at the bottom i wanna add $5 when i hit shift & / together, and wanna add $100 when i hit the ` key. But what i really don't know is how to add exactly THAT random amount on the bottom left to the jackpot on the top left, and make it what you see on the top right. I searched everywhere for that but i only got how to add whatever something is worth to the score and nothing else. And i'm asking this from a Unity and powerpoint and/or any game file software.

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