CREATE TABLE table_53636 ("City" TEXT, "1970" FLOAT, "1980" FLOAT, "1990" FLOAT, "2000" FLOAT, "2010 est." FLOAT)

Of 2010 est. with less than 171,750 and 2000 less than 131,714, what is the 1970 population average?
SELECT AVG("1970") FROM table_53636 WHERE "2010 est." = '171,750' AND "2000" < '131,714'
SELECT AVG("1970") FROM "table_53636" WHERE "2000" < '131,714' AND "2010 est." = '171,750'