one confusing probability

primefang

there are two totally same sets,that is(1,2,3,4,5),then select one number from every set specifically,ask for the probability that the sum of the 2 numbers is less than 4?

please give me relatively detailed explanation! forgive me for writting English(have to),thanks!

I count as 3/25, some claim it should be 5/25,

why?

tongxun

应该3/25。（1，2），（2，1）（1，1）。
选5/25。是因为没读懂less than 4。把1+3也算进去了。

primefang

thanks a lot