Q3.Consider four identical lifting wings of span 20 m flying at speed of 80 m/s and arranged in a line with a separation between adjacent wing tips of 5m. Use the simple horseshoe wing vortex system to calculate the induced downwash velocity at the centre of each wing. Use the results to estimate by how much the induced drag on each wing is decreased in comparison with a single, isolated wing

In the solution provided, some of the vortices generate an upwash. It isn't clear to me how to distinguish these vortices from the others that generate a downwash.