After seeing the decimals in the question we can equate the equation as follows:
36*28*10/4*1*3
= 840

Answer is A

Lets just turn everything into a whole number to make it easier.

Remember, when moving decimals within an division or multiplication problem we have to abide by the following rules:

When multiplying we have to move the decimal in the opposite direction of the two figures we are multiplying to keep the problem true to its original form. Ex: .002 * 567=1.134 is the same as 2 * .567 =1.134 if we move the decimal to the right side of the multiplication symbol you have to do the opposite to the whatever is on the left side to keep the multiplication true. This is the opposite when we divide. Whatever we do in the numerator we do in the denominator. For example this problem:

(0.0036)(2.8)/(0.04)(0.1)(0.003)

I make the denominator all integers by moving a total of 6 places to the right. Denominator = 4*1*3

I then have to do the same in the numerator to make the equation true to its original form =36*280

36*280/4*1*3=840

In numerator we have total 5 digits after decimal and in denominator we have 6 digits after the decimal so to avoid dealing with decimals we can multiply numerator and denominator by 10^6 which will give us 36*28*10/12 and further solving this we will get 840 so A is the Answer.

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Useful concept: \(\frac{A \times B \times C}{D \times E \times F} = \frac{A}{D} \times \frac{B}{E} \times \frac{C}{F}\)

Rearrange the given expression as follows: \(\frac{(0.0036)(2.8)(1)}{(0.003)(0.04)(0.1)}\)

Rewrite to get: \(\frac{0.0036}{0.003} \times \frac{2.8}{0.04} \times \frac{1}{0.1}\)

PRO TIP: If you're having difficulty some of the individual fractions, recognize that we can create equivalent fractions by multiplying numerator and denominator by a power of 10.

For example, we can take \(\frac{0.0036}{0.003}\) and multiply numerator and denominator by \(10,000\) to get: \(\frac{36}{30}\), which simplifies to \(\frac{12}{10}\), which equals \(1.2\)

Similarly, we can take \(\frac{2.8}{0.04}\) and multiply numerator and denominator by \(100\) to get: \(\frac{280}{4}\), which equals \(70\)

And we can take \(\frac{1}{0.1}\) and multiply numerator and denominator by \(10\) to get: \(\frac{10}{1}\), which equals \(10\)


So our expression becomes: \(1.2 \times 70 \times 10\), which equals \(840\)

Answer: A
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