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.)
>>...By mistake he multiplies by 10 and divides by 5...<<
That's the same as doubling, or multiplying by 2, because 10/5 = 2
>>...instead of multiplying by 5 and then dividing by 10...<<
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