Step 1. The party will be thrown by Steve, Jo, Lucy, Max, and Alex --> 5 people.
On Monday, each invites 3 friends. So for, the number of people invited is:
Step 2. On Tuesday, the friends that were invited (15) invited 3 more people each. The number of people invited now is:
Step 3. On Wednesday, the 45 people invited on Tuesday, each invite 3 more people. The number of people invited again multiplies by 3:
Step 4. The trend continues so for Thursday, the number of people invited on the previous day multiplies by 3:
Step 5. On Friday the number of people invited is:
Step 6. Finally, on Saturday, the day of the party, the number of people invited is multiplied one last time by 3:
Answer:
3,645