Someone is writing the answer so ill just give you this tip for whenever.
A negitive times a negitive is positive
a negative times a positive is negitive
and a positive times a positive is positive (just had to add it to be complete)
The answer is C <<<=====
It can't be anything else. It is not a right angle. A>90
It can't be acute because one angle is greater than 90.
It can't scalene because at least 2 of the angles (and 2 sides) are equal.
270,275. The answer would be the last choice.
Given that normally distributed data set has a mean of 55 and 99.7% of data fall between 47.5 and 62.5.
Let s be the standard deviation of data set.
Since 99.7% data fall within 3 standard deviations of mean, z-value for 47.5 and 62.5 has an absolute value of 3.
That is |z|=3
But z= 
Let us plugin x=47.5 and mean =55 and equate it to 3.
That is 

Since x is always positive ( being standard deviation), 
Hence 

We will get same value with 62.5 as well.
Hence standard deviation of data set is 2.5.