Question

Which number produces a rational number when multiplied by 0.4

Answer 1

A, B and D are already rational numbers, 0.4 itself is a rational number, two rational numbers make a product of rational number. 
So A, B, D are the correct choices. 

Are you sure it asks for rational numbers, not irrational number?

Answer 2

Answer:

A. $\frac{2}{7}$

B. $0.444....$

D. $\sqrt{9}$

Step-by-step explanation:

A rational number is a real number that can be expressed as $\frac{p}{q}$

Where, p and q are any integers,

Such that q ≠ 0,

If a real number is not rational then it is irrational.

Also, the product of two rational number is always a rational number while the product of a rational number and an irrational number is always an irrational number,

And, a non terminating recurring decimal and square root of a perfect square number are always a rational number,

Here, $\frac{2}{7}$, 0.444....., √9 and 0.4 are rational number,

⇒ 0.4 × 2/7, 0.4 × 0.4444...  and  0.4 × √9 are rational number.

Therefore, option A, B and D are correct.