n1-standard-1 is 1vCPU and 3.75GB of ram.
As per the documentation pricing per CPU and per GB of ram it should be:
0.031611 /vCPU hour - 0.031611 * 1 vCPU * 24 hours * 30 days = 22.75
0.004237 /GB hour - 0.004237 * 3.75 GB Ram * 24 hours * 30 days = 11.43
That's around 34 dollars.
But the GCP calculator and the monthly costs in the same documentation shows as 24$ per month. Is it due to sustained use discounts?