The average age of a class is 40 years. 12 new students with an average age of 32 years join the class, thereby decreasing the average age by 4 years. The original strength of the class was:

Let the original strength of the class be x. The original total age is 40x, and the total age of the 12 new students is 12 times 32, which is 384. The new average is 36, so the equation is 40x plus 384 equals 36 times the sum of x and 12. Solving 40x plus 384 equals 36x plus 432 gives 4x equals 48, meaning x is 12.

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