Question on Basis of Replacement property at future sale

[Ray Slack]

Example.  I bought a property for 100k and sold it for $500k  so, I have $400k profit.

I 1031 the property  above property and buy 2 properties one for $400k and one for $100k. 

a year later sell the $100k property for $150k

How would my basis in the property  I just sold for $150k be calculated? (Not talking about including expenses and selling costs and stuff)

I'm thinking it would be transferred proportional to the original basis I had in the first property I 1031ed.  So, about $20k basis give or take? 

I hope this makes sense.