单项选择题 A child withdraws from his piggy bank 10% of the original sum in the bank. If he must add 90￠ to bring the amount in the bank back up to the original sum, what was the original sum in the bank ？（）

单项选择题 The width of a rectangle is 6 cm less than the length. If the perimeter of the rectangle is 48 cm, what is the length of the rectangle in centimeters ？（）

单项选择题 A box contains 100 balls, numbered from 1 to 100. If three balls are selected at random and with replacement from the box, what is the probability that the sum of the three numbers on the balls selected from the box will be odd ？（）