When comparing 1450 and 938, the least precise of the 2 numbers is 1450 (it is correct to the nearest 10) so we need to write the answer correct to the nearest 10. We are using the result from above:

When adding or subtracting approximate numbers, the result should have the precision of the least precise number.

So we have

1450 − 938 = 512

Now 512 correct to the nearest 10 is 510. So the required answer is 510 km/h.

Be careful, though, since sometimes the result may look a bit silly.

Following this thinking, we would write

10 − 7 = 3 ≈ 0,

since "10" is written correct to the nearest 10 and the answer, 3, to the nearest 10 is 0.