The product of which expression contains four decimal places? (a) 5.3478x6.4214 (b) 6.4x3.82 (c) 521x3.82 (d) 14.2x0.784

Answer 
(D) 14.2x0.784

Explanation
We do not have to multiply these number to tell the number of decimal places of the product.
The product of a set of number will have the sum of the decimal places of the factors. For example, in 
(A) 5.3478x6.4214 ⇒ 4+4= 8 decimal places 
(B) 6.4x3.82           ⇒1+2= 3 decimal places
(C) 521x3.82          ⇒ 0+2 = 2 decimal places. 
(D) 14.2x0.784       = 1+3= 4 decimal places. 

the correct answer is (D) 14.2x0.784