You just got to test all the possible products

1 × 2 3 4 \large{\begin{array}{ccccccc} && & & & & 1 & \square\\ \times && & & & & 2 &\square\\ \hline & & & & & 3 & 4 &\square\\ \hline \end{array}} The above shows an incomplete long multiplication. Each square represents a single digit.

A possible way to complete this is 17 × 20 = 340 17\times20=340 . Is this the only possible way?

Yes No

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3 solutions

Naren Bhandari
Dec 5, 2018

Two more solutions exist 15 × 23 = 345 , 12 × 29 = 348 15\times 23 =345 , \ 12 \times 29 = 348 Writing problem as 1 A × 2 B = 34 C 10 ( 2 A + B ) + A B = 140 + C 1A \times 2B = 34C \implies 10(2A + B) +AB= 140 +C Observe that 140 10 ( 2 A + B ) + A B < 150 14 2 A + B + A B 10 < 15 140\leq 10(2A+B) +AB < 150\implies 14 \leq 2A+B+ \dfrac{AB}{10} < 15 As the largest possible value of A = 7 A=7 giving us 0 B < 1 B = 0 0\leq B < 1\implies B=0 and since 10 = 2 × 5 10=2\times 5 and if B = 5 B=5 then A A isnot integer (from above inequality ). Therefore A = 5 A=5 and then 4 B + B 2 < 5 8 3 B 10 4 \leq B+\frac{B}{2} < 5\implies 8 \leq 3B \leq 10 . Giving only possible B = 3 B=3 . For B = 2 B=2 does not hold good for 60 11 A < 65 60\leq 11A<65 meaning A = 2 A=2 and 10 B + B 5 < 11 50 6 B < 55 10\leq B +\frac{B}{5} <11\implies 50\leq 6B<55 giving us next possible value B = 9 B=9 .

Thus, 17 × 20 = 340 , 15 × 23 = 345 , 12 × 29 = 348 17\times 20= 340 \ , 15\times 23 = 345, \ 12 \times 29= 348 and the answer is yes

I chose "No" and it said "Incorrect"...

Eliyahu Tauber - 2 years, 1 month ago
Jordan Cahn
Dec 3, 2018

15 × 23 = 345 15\times 23 = 345

12 × 29 = 348 12 \times 29 = 348 is the third and final possible way.

Brian Charlesworth - 2 years, 6 months ago
Padmanavo Bhuiyan
Nov 17, 2020

15 23=345 and 12 29=348

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