Each table needs 1 plank of wood * 4 legs = <<1*4=4>>4 planks of wood for the legs.
This means each table needs 4 planks of wood for legs + 5 planks of wood for the surface = <<4+5=9>>9 planks of wood.
To build 5 tables, Johnny will therefore need 9 planks of wood * 5 tables = <<9*5=45>>45 planks of wood.
#### 45