Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| PRR Class | Builder’s Model | Build date | Total produced | Wheel arrangement | Service | Power output |
| --- | --- | --- | --- | --- | --- | --- |
| FS10 | H10-44 | 1948–1949 | 55.0 | B-B | Switcher | hp (kW) |
| FS12 | H12-44 | 1952–1954 | 16.0 | B-B | Switcher | hp (kW) |
| FF20 | t Erie Buil | 1947–1948 | 36.0 | A1A-A1A | Freight | hp (kW) |
| FF20 | Erie Built | 1947–1948 | 12.0 | A1A-A1A | Freight | hp (kW) |
| FF16 | CF-16-4 | 1950 | 16.0 | B-B | Freight | hp (kW) |
| FF16 | CF-16-4 | 1950 | 8.0 | B-B | Freight | hp (kW) |
| FRS-16 | H16-44 | 1952 | 10.0 | B-B | Freight | hp (kW) |
| FRS-20 | H20-44 | 1948–1951 | 38.0 | B-B | Freight | hp (kW) |
| FRS-24 | H24-66 | 1948–1951 | 38.0 | C-C | Freight | hp (kW) |
Question:
When was the build date for ff20 PRR class and erie built builder's model?
Answer:
1947–1948