What is the remainder when (2^16)(3^16)(7^16) is divided by 10? : Problem Solving (PS)

What is the remainder when is divided by 10?

A. 0
B. 2
C. 4
D. 6
E. 8

achloes wrote:ScottTargetTestPrep BrushMyQuant Bunuel chetan2u yashikaaggarwal

I was able to arrive at the answer without breaking apart the denominator 10 into 2 x 5.

However, when I do break it apart: (2^15 x 3^16 x 7^16) produces units digits of 6, and 1 and 1. But dividing the product by the denominator 5 produces a remainder of 1.

Could anyone help me understand why this method doesn't work?

Hey achloes

While other experts whom you've tagged to the post respond, sharing my two cents if that helps.

Before we figure out the reason why you're seeing a difference in answer let's take a general example to understand the concept -

What's the remainder when 10 is divided by 12 ?

The remainder () = 10, we can reason this well enough using logic right.

Now, let's say we divide the numerator and the denominator by 2

Remainder () = Remainder () = 5

Is the remainder when 10 divided by 12, 5? Of course, not!

What's the difference, the remainder got divided by 2 as well. In the first case, the remainder is 10, and in the second case, the remainder is 5.

The general rule is: Whenever you divide the numerator and denominator by a common factor, the common factor has to be multiplied (accounted for) to obtain the net remainder.

In our example, the common factor is 2.

Remainder () = Remainder () = 5

The final remainder = 5 * 2 = 10. This is what we're getting in the earlier example.

Now let's extend this concept to the question -

What is the remainder when is divided by 10 ?

If we find the unit's digit and use the concept of cyclicity we will get the unit digit of the product = 6 and that's our reminder. I guess you were able to get the answer without cancellation.

Now, let's see what happens when you cancel 2 from the numerator and denominator

Remainder ()

= Remainder ()

Using the concept of cyclicity, we can find the units' digits

= Remainder () = 3

Just as in the previous example, the remainders are no longer the same. In fact, the new remainder is half of the original remainder and this is expected

So to get the actual remainder, we need to multiply the obtained value by the factor used, i.e. multiply the new remainder by 2 to get the actual remainder.

3 * 2 = 6

Now by both methods, the remainders are the same.

Hope this help.
_________________

We need to find What is the remainder when is divided by 10

= = =

Now, we have split 42 into two numbers, one (40) is a number closer to 42 and a multiple of 10 and other is a small number

Now, if we expand this using Binomial theorem then we will get all terms except the last term as a multiple of 40 => A multiple of 10

=> All terms except the last term will give us a remainder of 0 when divided by 10

=> Remainder of by 10 is same as remainder of the last term = 16C16 * 2^16 * 40^0 = 2^16 by 10

Theory: Remainder of a number by 10 is same as remainder of the unit's digit of that number by 10

Now, Let's find the unit's digit of first.

We can do this by finding the pattern / cycle of unit's digit of power of 2 and then generalizing it.

Unit's digit of = 2
Unit's digit of = 4
Unit's digit of = 8
Unit's digit of = 6
Unit's digit of = 2

So, unit's digit of power of 2 repeats after every number.
=> We need to divided 16 by 4 and check what is the remainder
=> 16 divided by 4 gives 0 remainder

=> will have the same unit's digit as = 6
=> Unit's digits of = 6

But remainder of by 10 = 6

So, Answer will be D
Hope it helps!

Learn How to Find Remainders with 2, 3, 5, 9, 10 and Binomial Theorem

ScottTargetTestPrep BrushMyQuant Bunuel chetan2u yashikaaggarwal

I was able to arrive at the answer without breaking apart the denominator 10 into 2 x 5.

However, when I do break it apart: (2^15 x 3^16 x 7^16) produces units digits of 6, and 1 and 1. But dividing the product by the denominator 5 produces a remainder of 1.

Could anyone help me understand why this method doesn't work?