Are these HCG numbers okay?

Ka

6w0d Thursday 8/29 18,000 HCG

6w5d Tuesday 9/3 50,125 HCG

The nurse said it’s good and they’re doubling but when we hung up I wondered if it’s doubling like this

5 days between blood draws (so looking for 2-3 doublings)

18,000 x 3 = 54,000

Or

is it 18,000 x 2 = 36,000 x 2 = 72,000 x 2= 144,000

Yesterday we heard and saw the HB and it was 120 BPM