Accelerations do not add. Masses do.
A resultant force of 20 N gives a body of mass m an acceleration of 8.0 m/s², and a body of mass m′ an acceleration of 24 m/s². What acceleration will this force cause the two masses to acquire if fastened together?as printed, page 52
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A bag of sugar and a bit. Twenty newtons gets it to 8 m/s in one second.
A litre of water, nearly. Same push, three times the acceleration, because a third the mass.
Not 8 plus 24, not the average. Add the masses, then divide. The pair is heavier than either, so it accelerates less than either.
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