[Two dimensional chart - see screenshot in link below. Heading: The Klein Family's Annual Income, 1985-1995. X-axis: year, from 1985 to 1995. Y-axis: income, from 20,000 to 70,000 with lines at 5,000 increments]
http://www.postimage.org/image.php?v=gx1m4OW9

Which of the following statements can be inferred from the data above?
I. The Klein family's annual income more than doubled from 1985 to 1995.
II. The Klein family's annual income increased by a greater amount from 1985 to 1990 than from 1990 to 1995.
III. The Klein family's average (arithmetic mean) annual income for the period shown was greater than $40,000.

a. I only
b. II only
c. I and III only
d. II and III only
e. I, II, and III

Original Answer is C.
Could anyone help explain why?

you can solve these by reading the incomes off the graph. for each year, the income is the y-value for that year.

(i)
the income for 1985 is about $25,000.
the income for 1995 is over $60,000, which is well over twice $25,000.
so, this statement is true.

(ii)
the income for 1985 is about $25,000.
the income for 1990 is between $35,000 and $40,000, closer to $40,000. let's say $38,000.
the income for 1995 is between $60,000 and $65,000, closer to $65,000. let's say $63,000.
the increase from '85 to '90 is about $13,000; the increase from '90 to '95 is about $25,000. there is no question that the second is greater.
thus, statement (ii) is false.

(iii)
this problem is the time-suck; the easiest way to do it is to add all the incomes together and divide by eleven (the total number of years for which income is shown).
let's estimate (in thousands - i'm not going to type all the ",000"s, and you shouldn't write them on test day either):
(25 + 26 + 26 + 35 + 36 + 38 + 38 + 57 + 63 + 63 + 63) / 11
= 470/11
= comfortably greater than 40 (it's 42 and change)
so, this statement is true.

unfortunately, not a lot of shortcuts here.

Thanks! I approached it the same way you did, and just realised I made a calculation error.

if you made a calculation error, be sure to take inventory of whatever error(s) you made, so that you can be on "extra lookout" for the same type of error in the future.

usually, the arithmetic errors you make will fit some sort of narrow pattern or description - i.e., it's rare for your errors to be completely random - so, if you take inventory, you'll become aware of the particular types of errors that you tend to commit most often.