Question 1133239: Twenty girls stood in a row facing right. Four boys joined the row, but facing
left. Each boy counted the number of girls in front of him. The numbers were 3,
6, 15 and 18 respectively. Each girl also counted the number of boys in front of
her. What was the sum of the numbers counted by the girls?
Answer by ikleyn(52803)   (Show Source):
You can put this solution on YOUR website! .
It is easy.
From the condition (from the given data), you can restore the placement of the girls and the boys in the row, from left to right.
It is as follows:
G1 G2 G3 B1 G4 G5 G6 B2 G7 G8 G9 G10 G11 G12 G13 G14 G15 B3 G16 G17 G18 B4 G19 G20
Then the number of boys counted by each girl in front of she, is as follows:
G1 G2 G3 G4 G5 G6 G7 G8 G9 G10 G11 G12 G13 G14 G15 G16 G17 G18 G19 G20
4 4 4 3 3 3 2 2 2 2 2 2 2 2 2 1 1 1 0 0
So, the sum you are looking for is 4*3 + 3*3 + 2*9 + 1*3 + 0*2 = 42.
Now, having this solution before your eyes, it is not difficult to re-tell it in the compact form, by writing
and justifying the last summation immediately, omitting all details that preceed.
Solved, answered, explained and completed.
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