Target Test Prep Sample Units Digit Pattern Questions
Example 1
Solution:
The pattern for the units digits of powers of 3 is 3–9–7–1. This means that all powers of 3 with a multiple of 4 in the exponent will have a 1 as the units digit. A multiple of 4 that is close to 247 is 244. This means that the units digit of 3244 is a 1, that of 3245 is a 3, that of 3246 is a 9, and that of 3247 is a 7.
Example 2
If x and y are positive integers, what is the units digit of
1)
2)
Solution:
Question Stem Analysis:
The units digits of powers of 7 follow the four-number pattern 7–9–3–1. We need to determine the units digit of
Statement One Alone:
⇒
We can restate this as x = 16n. Because n is an integer, it must be true that x is a multiple of 16. This means that xy is a multiple of 16 because the product of 16 and any positive integer is a multiple of 16. Because 16 is a multiple of 4, we also know that xy is a multiple of 4. However, because we do not know anything about y, statement one alone is not sufficient.
Eliminate answer choices A and D.
Statement Two Alone:
⇒
We can restate this as y = 8m. Because m is an integer, it must be true that y is a multiple of 8. This also means that xy is a multiple of 8. We can now think about this as
Example 3
Solution:
Since each answer choice has a different units digit, instead of finding the actual sum we can just find the units digit of the sum. Let's use the units digits of each square to determine the units digit of the sum.
⇒ The units digit of 202 must be 0, since 02 = 0.
⇒ The units digit of 212 must be 1, since 12 = 1.
⇒ The units digit of 222 must be 4, since 22 = 4.
⇒ The units digit of 232 must be 9, since 32 = 9.
⇒ The units digit of 242 must be 6, since 42 = 16.
⇒ The units digit of 252 must be 5, since 52 =25.
With this, we can sum the units digits: 0 + 1 + 4 + 9 + 6 + 5 = 25. Thus, the units digit is 5.