求助 数学 OG12 DS 167

一只追梦的猫

167：If n is positive integer,what is the tens digit of n?
(1)The hundreds digit of 10n is 6.
(2)The tens digits of n+1 is 7.
偶素文科生，数学不太好，求高人指点，谢谢啦

sdcar2010

n = 10*a + b; what is a?

1) 10*n = 6*100 +b'*10.  If so, then n = 6*10 + b', so a = 6. Sufficient.
2) n+1 = 10*7 + b'. If so, then n = 10*7 +b'-1. If b'=0, then n=10*6 +9, a=6. But if b'=1,2,3, ...9, then n=10*7 + (b'-1), a = 6. In sufficient.

AAAAAAAAAAAAAAa

sdcar2010

Another way to analyze the second condition is to use REAL number. Let n+1 equal 70 and 71, what is the value of a? a=6 for the former while a = 7 for the latter. In sufficient.

一只追梦的猫

两道数学题都是你回复的，谢谢你的帮助啦，很热心

sdcar2010

I love brain exercises.