A/B is a reduced fraction which can be represented as a terminating decimal if and only b is of the form 2^n5^n where m and n are non negative integers. For example: 7/250, the terminating decimal here is 0.028 as 250 as the denominator equals to 2*5^2.
According to the above, we must know whether the denominator have only 2-s and or 5-s as the prime factors. So,
Q = 8 = 2 ^3 = hence the denominator has only 2 prime factors. Fraction P/Q will be termed as terminated decimal which is sufficient.