It would take the assistant 20 hours to do the job alone.
From the information given, we have that;
Let the gardener be x
His assistant be y
So,
y = 1/ 5
x + y = 1/ 4
Substitute the value
x = 1/ 4 - 1/ 5
Find the LCM
x = 1 / 20
Take the reciprocal
x = 20 hours
Thus, it takes the assistant 20 hours to do the job alone.
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