First, we need to calculate how much money Anna saved in total.
She saved $20 per month for 2 1/2 years, which is 30 months (2.5 years x 12 months per year).
Total savings = $20/month x 30 months = $600
Next, we need to calculate how much money Anna spent on the computer.
75% of $600 = $450
Finally, we can calculate how much money Anna had left in the jar:
Total savings - Money spent on computer = $600 - $450 = $150
Therefore, Anna had $150 left in the jar.