The mean is 10,5

n=16

Standart deviation is 2,3

We want to know if we can say that the mean will be higher than 10 with what risk of error?

the solution is :

10,5-10=0,5, so 0,5=ta/2*2,3/4, and a=40% so the risk of error is 40%.

I do not understand what the calcul is how do they find 40%?