kitchen table math, the sequel: fun SAT Question of the Day

## Monday, April 25, 2011

### fun SAT Question of the Day

What is the largest possible integer value of n for which 5n divides 507?

(A) 2
(B) 7
(C) 9
(D) 10
(E) 14

Richard I said...

Ahhhhhh ..... prime factorisation where 2 and 5 are the only prime factors.

Takes me back to a grade 8 Honours class I once taught where we took recurring and terminating decimals to pieces. There's nothing those kids couldn't do ....

Stuart Buck said...

14? (Because 50^7 = 5^7 * 10^7 = 5^7 * 5^7 * 2^7 = 5^14 * 2^7?)

Catherine Johnson said...

I was happy about this problem because just a few months ago I would have been mystified and today I could 'see' how to do it....

Catherine Johnson said...

This is something I like so much about SAT prep (assuming you're doing SAT prep the way we're doing SAT prep, which is to learn how to do the problems as opposed to learning 'strategies').

Every time I take an SAT section, I discover 'glitches' in my algebra 1 knowledge. These aren't gaps, exactly (for passersby: we've talked about gaps a lot on the blog).

It's not that I don't 'know' the concepts and procedures covered by the test.

It's that I don't know - or see? - the implications of what I know.

"Glitches" feels like the right word for the phenomenon.

In any event, having this chance to de-glitch C. (& me) as much as I can is great.

Anonymous said...

Oh yeah! I was just about to look for an SAT problem where the answer was trivial if you used prime factorization for one of my tutoring students.

50^7 = (5*5*2)^7
= (5^7)(5^7)(2^7)
=(5^14)(2^7)

Good question.