How many integers between 324,700 and 458,600 have tens digit 2 and units digit 1?

AAPL wrote: ↑

Mon Nov 08, 2021 5:22 pm

GMAT Prep

How many integers between 324,700 and 458,600 have tens digit 2 and units digit 1?

A. 1,339
B. 1,352
C. 1,353
D. 10,030
E. 10,300

OA A

A nice rule says: the number of integers from x to y inclusive equals y - x + 1
So, the number of integers from 324,700 to 458,600 = 458,600 - 324,700 + 1 = 133901
ASIDE: We can safely round this number 133900 (you'll see why shortly)

Now let's look at how frequently we have a 1 in the tens digit and 3 in the units digit.
Integers from 1 to 100: 13 (1 integer)
Integers from 101 to 200: 113 (1 integer)
Integers from 201 to 300: 213 (1 integer)
Integers from 301 to 400: 313 (1 integer)
.
.
.
As we can see, for every 100 integers, there's 1 integer with a 1 in the tens digit and 3 in the units digit.
In other words, 1/100 of all integers have a 1 in the tens digit and 3 in the units digit.

So, among the 133900 integers in question 1/100 of them meet the given condition.
(1/100)(133900) = 1339

Answer: A
Cheers,
Brent