Problem Solving

What is the unit digit of the sum 3^47 + 5^43 + 2^12?

A) 8
B) 4
C) 9
D) 1
E) 3

The OA is A.

Can any expert help me with this PS question please? Thanks.

What is the unit digit of the sum 3^47 + 5^43 + 2^12?

A) 8
B) 4
C) 9
D) 1
E) 3

The OA is A.

Can any expert help me with this PS question please? Thanks.

Hi LUANDATO,
Let's take a look at your question.

We will first look at some of the exponents of 3, 5 and 2 to find out the pattern of units digit in each number.

$$3^1,\ 3^2,\ 3^3,\ 3^4,\ 3^5,\ 3^6,\ ...$$
$$=3,\ 9,\ 27,\ 81,\ 243,\ 729,...$$
We can see that in the powers of 3, the unit digit repeats itself after every 4 numbers in the sequence.

Let's now examine the pattern of 5
$$5^1,\ 5^2,\ 5^3,\ 5^4,\ 5^5,\ 5^6,\ ...$$
$$=5,\ 25,\ 125,\ 625,\ ...$$
Unit digit is 5 for all the exponents of 5.

Let's now examine the pattern of 2
$$2^1,\ 2^2,\ 2^3,\ 2^4,\ 2^5,\ 2^6,\ ...$$
$$=2,\ 4,\ 8,\ 16,\ 32,\ 64,\ ...$$

Now using these patterns we will find out the unit digit of 3^47.
Since unit digit in exponents of 3's sequence repeats after every 4 exponents.
47th power = 4(11) + 3 => 3rd exponent's unit digit in 3's sequence is 7

For all the exponents of 5 unit digit is 5, so unit digit of 5^43 is 5.

Now, we will find out the unit digit of 2^12.
Since unit digit in exponents of 2's sequence repeats after every 4 exponents.
12th power = 2(6)+0 => last exponent's unit digit in 2's sequence is 6

Let's now add up all the unit digits = 7 + 5 + 6 = 18
Hence, the unit digit is 8.

Therefore, Option A is correct.

Hope it helps.
I am available if you'd like any follow up.