if you were asked to make a magnitude estimate of 77.7?

Answer 1

To make a magnitude estimate you replace the number by a "simpler" number.

The simpler number being typically a something like 50000,
400, 10000 (i.e. one non null digit followed by or or more Zeroes)

For 77.7 the approximate is then 80

Note the approximate is NOT 70 because with 70 you make an error of 7.7 while with 80 you make a smaller error of 2.3 .

Answer 2

Magnitude is simply the comparison of two numbers. You only gave us one number, 77.7. When we say "an order of magnitude", we ususally mean one place to the log of base 10. So if your number is 77.7 and the original is 7.77, we would say that your number is an order of magnitude greater. If the original was 777, then your number is an order of magnitude smaller.

Magnitude requires two numbers to be compared, just like logarithms. Without knowing both numbers, we can't help you further.