a baker's dozen

[Kati Svaby]

English Language Tests, Intermediate level

ESL/EFL Test #281 "English Slang Idioms (3)", question 8

The little boy asked his mother why a baker's ......... was thirteen. She replied that it comes from the former custom among bakers of adding an extra roll as a safeguard against the possibility of 12 weighing light.

(a) dozen
(b) product
(c) quantity
(d) delivery

English Language Tests, Intermediate level

ESL/EFL Test #281 "English Slang Idioms (3)", answer 8

The little boy asked his mother why a baker's dozen was thirteen. She replied that it comes from the former custom among bakers of adding an extra roll as a safeguard against the possibility of 12 weighing light.

Correct answer: (a) dozen
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Baker's dozen /n/ /sing/ a group of thirteen(=one more than a dozen, which is twelve).
Origin: This phrase comes from baker's old custom of adding one extra loaf to an order of a dozen. (Oxford Dictionary)
If this explanation isn't good, please not to delete but to put an additional explanation.
Regards:
kati Svaby
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We always deceive ourselves twice about the people we love - first to their advantage, then to their disadvantage.