let

g = the speed of the glider in still air

w = the speed of the wind

a glider can fly 240 kilometers in 3 hours with the wind

(g + w)*3 = 240 divide both sides by 3

g + w = 80

against the wind the glider can only fly 210 km in 3 hours

(g - w)*3 = 210 divide both sides by 3

g - w = 70

by solving the system of equations

g + w = 80

g - w = 70

we find

g = 75 km/h

w = 5 km/h

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the speed of the glider in still air is 75 km/h.

the speed of the wind is 5 km/h.