Well it all depends what measurement your answer has to be in.
Answer in cm-
12cm stays the same
143m becomes 14,300cm
398km becomes 39,800,000cm
778mm becomes 77.8cm
Then you add them together
12+14,300+39,800,000+77.8=39,814,398.8
The answer stays the same cause it already has the right amount of sig figs so it's 39,814,398.8cm
Answer in m-
12cm becomes 0.12m
143m stays the same
398km becomes 398,000m
778mm becomes 0.778m
Then you add them together
0.12+143+398,000+0.778=398,143.898
But then you have to round to the right amount of sig figs which would be to have two numbers after the decimal point so the answer would then be 398,143.90m
Answer in km-
12cm becomes 0.00012km
143m becomes 0.143km
398km stays the same
778mm becomes 0.000778
Then you add them together
0.00012+0.143+398+0.000778=398.143898km
But then you have to round to the right amount of sig figs which would be to have three numbers after the decimal point so the answer would then be 398.144km
Answer in mm-
12cm becomes 120mm
143m becomes 143,000mm
398km becomes 398,000,000mm
778mm stays the same
Then you add them together
120+143,000+398,000,000+778=398,143,898
The answer stays the same cause it already has the right amount of sig figs so it's 398,143,898mm
