what should I answer with
"How's your day?"
"How's It going?"
example plz

can I say "pretty good"?

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Yes, you can answer "pretty good." :)

Or, other replies are~

(How's your day)
"It's been great, and yours?"
"It's been good, yours?"

(How's it going)
"I'm well, how about you?"
"It's going great, what about you?"

^ these are all happy/positive replies, but you can also say "I've had better days" or "It's not been great.."