GWD第25套有3道数学题请教，请大家帮忙～～～

the sides of a square region, measured to the nearest centimeter, are 6 centimeters long. the least possible value of the actual area of the square region is?
我认为是30.25，可是答案给了33.75？？

if the average (arithmetic mean) of the assessed values of  x housed is $212000, and the average of the assessed values of y other houses is $194000, what is the average of the assessed values of the x+y houses?
(1）x+y=36
(2) x=2y
我选了B，其实把B代进去，就可以把字母都去掉了，可是答案选 了C。

last year in a group of 30 business, 21 reported a net profit and 15had investments in foreign markets. how many of the businesses did not report a net profit nor inverst in foreign markets last year?
(1) last year 12 of the 30 businesses reported a net profit and had investments in foreign markets.
(2) last year 24 of the 30 businesses reported a net profit or invested in foreign markets, or both.
我选择了D，但是答案给了C。理解不了。

请求高人指导，不胜感激～～～～