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minor arc CB =  180  - minor arc AC  =  180 -  40 =  140

(1/2) (minor arc AD + minor arc CB)  = angle AED

(1/2) (100 + 140)  =  120° = angle AED

angle AED + angle DEB =  180

120  + angle DEB =  180

angle DEB  = 180 - 120   =   60°