5.2, 2.1 and 4.5 "terminate." They are rational numbers. On the other hand, 1/7 is a proper fraction, but its decimal equivalent does not terminate: 0.142857142....
It would be faster to approximate the answer by converting 1/7 to a decimal fraction and round off the final result to 1 decimal place.
Certainly we could rewrite the original expression 5.2 + 2.1(4.5 - 4 1/7 ) as
5 2/10 + (2 1/10)*(4 1/2 - 4 1/7), find the LCD, and then go through the indicated operations, to obtain an "exact" answer. But given that all of the original numbers are given to only 1 decimal place, it makes no sense to worrry about accuracy to 2, 3, 4 or more decimal places. I'm with Josie.
