A joint survey by two conservation groups, World Conservation Union and the World Wildlife Fund, says the black rhino population has risen to over 3,600. That is an increase of 500 animals over the last two years.
two years ago the black rhino population was over 3100
If the rhino population has increased in the last 2 years by 500 to over 3600 then it follows that there were over 3100 (3600-500) two years ago. The system could not reason this out as it is a complex concept
( z1 over :op ( z2 population :quant 3100 :cxnsistof ( z3 rhino :ARG1-of ( z4 black ) ) ) :time ( z5 before :op ( z6 now ) :quant ( z7 temporal-quantity :quant 2 :unit ( z8 year ) ) ) )