SOLUTION: Dou is working on a math problem in class, By mistake he multiplies by 10 and divides by 5 instead of multiplying by 5 and then dividing by 10. If the incorrect answer is 3, what i

The answer may not be an integer.  Never just assume that every answer has to
come out to be a positive whole number.  You should not rule out negative or
fraction answers unless you can tell that the answer must be a positive whole number.)

That's the same as doubling, or multiplying by 2, because 10/5 = 2

That's the same as multiplying by 1/2, because 5/10 = 1/2

So he made the mistake of doubling it instead of halving it.  When
he doubled it he got 3.   So he had to have started with half of 3
which is 1 1/2 or 3/2.  So he should have halved that 3/2 and gotten 3/4.

Edwin