Thread: Q43. Math (DS) - Units digit and factorial

D

Units digit 0

A,
From the 1st we know that the value of x is at least 5, which means that X has as a factor 5 and 2, which makes the units digit equal 0.
from the 2nd we know nothing. The value can be either 1!, 2!, 3! until 49 and the unit digit will change until 5

Can someone help me with this one. I don't understand how both of your came to 0. Here is my explaination.
E – 20 secs
1.) X > 4: à 5,6,7,8,9,10,11… not sufficient
2.) X = 0,1,2,3,4,5,6,7,8,9 not sufficient
Together: 4 < x < 50 à units digit could be 5,6,7,8,9,0,..,9 not sufficient

a

Originally Posted by rkandell

Can someone help me with this one. I don't understand how both of your came to 0. Here is my explaination.
E – 20 secs
1.) X > 4: à 5,6,7,8,9,10,11… not sufficient
2.) X = 0,1,2,3,4,5,6,7,8,9 not sufficient
Together: 4 < x < 50 à units digit could be 5,6,7,8,9,0,..,9 not sufficient

You missed that the question asks not about the value of X, but the value of X!. Thus when it starts with 5! it will always have 5 and 2 factors.

I read right over the factorial sign. Thanks Clerk. I agree A is the answer.

A

Could I please ask all those who post answers to add at least a little hint about their solution? What's the sense in posting only answers especially when the answer is already evident or OE is posted?
I think such practice is useful for both posting people and people who receive mail notifications.

I agree 100% with Clerk. It is sometimes frustrating when people come up with different answers than you but leave out their explanation.