Whole Number P6 2011 PSLE

The chairs in a hall were arranged in rows of 9 chairs each. For a concert to be held in the hall, Alex bought another 6 chairs and rearranged the chairs into rows of 7 chairs each. After the rearrangment, he found that he had 12 additional rows. How many chairs were there in the hall for the concert?

12  x  7  =  84

84  -  6  =  78

9 - 7  =  2

78  ÷  2  =  39  rows originally

39  x  9  =  351

351 + 6  =  357 chairs


It was  357  chairs.


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Alternative solution
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9 x 1 u  -->  7 x 1 p - 6 
{1 u -- number of rows at the start; 1 p -- number of rows in the end}

9 u  -->  7 p  -  6

1 u  -->  1 p  -  12

7 u  -->  7 p  -  7 x 12  =  7 p  -  84

2 u  -->  84  -  6  =  78
1 u  -->  78  ÷  2  =  39

39  x  9  =  351

351  +  6  =  357


It was  357  chairs.