[Grammar] PLURAL/SINGULAR

1. One and a half hours are allowed for the exam.
2. An hour and a half is allowed for the exam.
   
   
   The above two examples have been taken out from the Oxford Advanced Learner's Dictionary.​

My question is that why those two sentences read differently when it comes to verb being singular or plural. Please explain. I can't really understand the double standard in the examples. The same verb with the same idea but in two different forms. Why?

Innocent Fairy said:

1. One and a half hours are allowed for the exam.
2. An hour and a half is allowed for the exam.
   
   
   The above two examples have been taken out from the Oxford Advanced Learner's Dictionary.​

My question is that why those two sentences read differently when it comes to verb being singular or plural. Please explain. I can't really understand the double standard in the examples. The same verb with the same idea but in two different forms. Why?

Click to expand...

An hour and a half = a recognised name for one period of time lasting 90 minutes, so that one period is treated as the singular.

I can't actually give you a logical explanation as to why this is though, unfortunately, but in BrE at least, that's just how it is.

Not a teacher.
Your first sentence is justified by.
http://www.englishclub.com/tefl/viewtopic.php?t=1584

Second sentence is justified by the following rule.
Quantities or measurements of time, money, distance, weight usually take singular verbs.

http://isu.indstate.edu/writing/esl/setvi/prints/SETVI3AP.html

Innocent Fairy said:

1. One and a half hours are allowed for the exam.
2. An hour and a half is allowed for the exam.
   
   
   
   The above two examples have been taken out from the Oxford Advanced Learner's Dictionary.​

My question is that why those two sentences read differently when it comes to verb being singular or plural. Please explain. I can't really understand the double standard in the examples. The same verb with the same idea but in two different forms. Why?

Click to expand...

Use a plural verb for more than one unit e.g. One and a half hours are...
EXCEPT when a determiner, e.g. an, is used for one unit or one unit plus a fraction thereof.