NTSE is the best

If a , b , a, b, and c c are positive integers satisfying the following equations:

5 a + 5 b + 2 a b = 92 5 b + 5 c + 2 b c = 136 5 c + 5 a + 2 c a = 244 , \begin{aligned} 5a+5b+2ab&=92\\ 5b+5c+2bc&=136\\ 5c+5a+2ca&=244, \end{aligned}

what is the value of b ? b?


The answer is 3.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Multiply the first equation by 2 2 throughout we have:

10 a + 10 b + 4 a b = 184 ( 2 a + 5 ) ( 2 b + 5 ) = 209 = ( 11 ) ( 19 ) 10a+10b+4ab = 184 \quad \Rightarrow (2a+5)(2b+5)=209=(11)(19)

Since a a and b b are integers, it means:

( 2 a ) ( 2 b ) = ( 11 5 ) ( 19 5 ) = ( 6 ) ( 14 ) a b = ( 3 ) ( 7 ) (2a)(2b)=(11-5)(19-5)=(6)(14)\quad \Rightarrow ab=(3)(7)

Similarly,

10 b + 10 c + 4 b c = 272 ( 2 b + 5 ) ( 2 c + 5 ) = 297 = ( 11 ) ( 27 ) 10b+10c+4bc = 272 \quad \Rightarrow (2b+5)(2c+5)=297=(11)(27)

b c = ( 3 ) ( 11 ) \Rightarrow bc=(3)(11)

And

10 c + 10 a + 4 c a = 488 ( 2 b + 5 ) ( 2 c + 5 ) = 513 = ( 19 ) ( 27 ) 10c+10a+4ca = 488 \quad \Rightarrow (2b+5)(2c+5)=513=(19)(27)

c a = ( 7 ) ( 11 ) \Rightarrow ca=(7)(11)

As both b b and 3 3 appear in Eqn 1 and Eqn 2, b = 3 b=\boxed{3}

Alternatively, we can deduce from the same set of equations that 2 a + 5 = 19 , 2 b + 5 = 11 , 2 c + 5 = 27 2a+5=19,2b+5=11,2c+5=27 and from there get b = 3 b=\boxed{3}

Jared Low - 6 years, 2 months ago
Utsav Banerjee
Aug 23, 2014

Subtracting pairs of equations, we get:

(c - a) * (2b + 5) = 44 or, c - a = 44 / (2b + 5)

(a - b) * (2c + 5) = 108 or, a - b = 108 / (2c + 5)

(b - c) * (2a + 5) = -152 or, b - c = -152 / (2a + 5)

Note that each of these 3 quantities is an integer as they are the differences of 2 integers, which must be integers. So, (2b + 5 ) | 44, (2c + 5) | 108, and (2a + 5) | 152

Since a > 0, b > 0, c > 0, we have 2a + 5 > 5, 2b + 5 > 5, 2c + 5 > 5

Now, 44 has factors 1, 2, 4, 11, 22 and 44. So, 2b + 5 can be equal to 11, 22 or 44. A little trial will yield the solution b = 3, a = 7, and c = 11

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...