The mother was also interested, and looked smilingly toward her bright son.  Like every mother, her sympathies went out to him.  When Nick told his father that he was in error, the mother felt a thrill of delight; she wanted Nick to get the better of her husband, much as she loved both, and you and I can’t blame her.

Nick leaned back in his chair, shoved his hands into his pockets, and looked smilingly at his father and his pipe as he said: 

“Suppose, to illustrate, that Philadelphia has just one hundred people.  Then, if New York has fifty per cent. more, it must have one hundred and fifty people as its population; that is correct, is it not, father?”

Mr. Ribsam took another puff or two, as if to make sure that his boy was not leading him into a trap, and then he solemnly nodded his head.

“Dot ish so,—­dot am,—­yaw.”

“Then if Philadelphia has one hundred people for its population, New York has one hundred and fifty?”

“Yaw, and Pheelatelphy has feefty per cent. less—­yaw, yaw, yaw!”

“Hold on, father,—­not so fast.  I’m teacher just now, and you mustn’t run ahead of me.  If you will notice in this problem the per cent. in the first part is based on Philadelphia’s population, while in the second part it is based on the population of New York, and since the population of the two cities is different, the per cent. cannot be the same.”

“How dot is?” asked Mr. Ribsam, showing eager interest in the reasoning of the boy.

“We have agreed, to begin with, that the population of Philadelphia is one hundred and of New York one hundred and fifty.  Now, how many people will have to be subtracted from New York’s population to make it the same as Philadelphia?”

“Feefty,—­vot I says.”

“And fifty is what part of one hundred and fifty,—­that is, what part of the population of New York?”

“It vos one thirds.”

“And one third of anything is thirty-three and one third per cent. of it, which is the correct answer to the problem.”

Mr. Ribsam held his pipe suspended in one hand while he stared with open mouth into the smiling face of his son, as though he did not quite grasp his reasoning.

“Vot you don’t laughs at?” he said, turning sharply toward his wife, who had resumed her knitting and was dropping many a stitch because of the mirth, which shook her as vigorously as it stirred her husband a few minutes before.

“I laughs ven some folks dinks dey ain’t shmarter don dey vosn’t all te vile, don’t it?”

And stopping her knitting she threw back her head and laughed unrestrainedly.  Her husband hastily shoved the stem of his pipe between his lips, sunk lower down in the chair, and smoked so hard that his head soon became almost invisible in the vapor.

By-and-by he roused himself and asked Nick to begin with the first problem and reason out the result he obtained with each one in turn.