Reverse Pascal Triangle

Algebra Level 2

Every brick in this pyramid contains a number that is the sum of the two numbers in the two bricks below it. What is the value of B B ?


The answer is 19.

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8 solutions

We will use the corresponding lowercase letters to the bricks assigned to them; the top layer is layer 1, the layer below is layer 2, and so on.

Layer 4 4 :

137 + b = f 137 + b = f

52 + b = g 52 + b = g

52 + d = h 52 + d = h

d + e = 147 d + e = 147

Layer 3 3 :

f + g = j 137 + 52 + 2 b = j 189 + 2 b = j f + g = j \rightarrow 137 + 52 + 2b = j \rightarrow 189 + 2b = j

g + h = 225 52 + b + d + 52 = 225 b + d = 121 g + h = 225 \rightarrow 52 + b + d + 52 = 225 \rightarrow b + d = 121

h + 147 = l 52 + d + 147 = l 199 + d = l h + 147 = l \rightarrow 52 + d + 147 = l \rightarrow 199 + d = l

Layer 2 2 :

j + 225 = m 189 + 2 b + 225 = m 414 + 2 b = m j + 225 = m \rightarrow 189 + 2b + 225 = m \rightarrow 414 + 2b = m

225 + l = n 225 + 199 + d = n 424 + d = n 225 + l = n \rightarrow 225 + 199 + d = n \rightarrow 424 + d = n

Layer 1 1 :

m + n = 978 2 b + d + 838 = 978 2 b + d = 140 m + n = 978 \rightarrow 2b + d + 838 = 978 \rightarrow 2b + d = 140

Now, we have a system of equations:

{ b + d = 121 2 b + d = 140 \begin{cases} b + d = 121 \\ 2b + d = 140 \end{cases}

Substituting the first equation in the second yields:

b + 121 = 140 b = 19 b + 121 = 140 \rightarrow b = 19 , which is the sought answer.

Christian Barrera
Jan 27, 2015

The format of the bricks will be similar to the Pascal's Triangle

From O and K:

O = A + 4B + 6C + 4D + E

K = B + 2C + D

Substituting given values we can get :

382 = 4B + 3D

121 = B + D

Given those two equations we get B = 19, D = 102

Amy Collinson
Jan 22, 2017

Writing the top layer in terms of the bottom layer:

978 = A + 4B + 6C + 4D + E

978 = A + B + 3(B + 2C + D) + (D + E)

978 = A + B + 3K + I

978 = 137 + B + (3x225) + 147

Therefore, B = 978 - 959

B = 19

Omkar Kulkarni
Jan 23, 2015

So the way to do this is to take B B as x x , and find all the other boxes in terms of x x and hope for your answer to come soon,

B = x B = x

First of all, F = 137 + x F = 137+x

G = 52 + x G=52+x

J = 189 + 2 x J=189+2x

M = 414 + 2 x M=414+2x

And then, K = G + H H = 173 x K=G+H \Rightarrow H = 173 - x

L = 320 x L=320-x

N = 545 x N=545-x

And finally. M + N = O x = 19 M+N=O \Rightarrow \boxed{x=19}

You can see there is a pattern of Pascal Triangle in this question.

AccelNano Lim Loong - 6 years, 4 months ago

@Owen R This was a fun question!

Calvin Lin Staff - 6 years, 4 months ago
Gamal Sultan
Jan 30, 2015

M + N = 978 = J + L + 2(225)

J + L = 528 ............................... (1)

G + H = 225 ..............................(2)

J + L + 225 = 2(G + H) + F + 147 .......................(3)

From (1), (2) and (3)

F = 156

F = 137 + B

B = 19

Jason Abalajen
Jan 28, 2015

Sameh Samir
Jan 27, 2015

G + H = 225, B - D = G - H, SINCE (D = 140 - 2B) AND (H = 225 - G), B - (140 - 2B) = G -(225 - G), 3B - 140 = 2G - 225, SINCE (B = G - 52), 3(G - 52) - 140 = 2G - 225 >>> G=71,

THE REST IS HISTORY.

Yosua Leonardo
Jan 27, 2015

F=137+B

G=52+B

J=189+2B

M=414+2B

O=414+2B+N


H=52+D

L=199+D

N=424+D

O=838+2B+D

2B+D=140


G+H=225

52+B+52+D=225

B+D=121


2B+D=140

B+D=121

--------(-)

B = 19

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