Divisibility of 11: If the difference of the sum of the digits at odd places and the sum of the digits at even places of a number is zero or a multiple of 11, then that number is divisible by 11.As per divisibility rule of 11, difference = (x + 21) - 18 = x + 3=> x + 3 = 11 => x = 8
Let the price of a pen be 'x' and that of pencil be 'y' Then 87x + 29y = 783Multiplying eqn (i) by 30/29 we get (87x) x 30/29 + (29y) 30/29 = 783 x 30/29∴ 90x + 30y = 810
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