Give the number of significant figures in this number: 7.8 x 10¹²

The answer would be:

Two significant figures

In scientific notation, the only significant numbers would be in the coefficient. In your case:

7.8 x 10¹²

7.8 is the coefficient and there are two numbers, 7 and 8. This means that they are the only significant numbers.

But take note:

ALL NUMBERS IN THE COEFFICIENT ARE SIGNIFICANT!

So if you write something like:

7.80 x 10¹² (which is technically the same as the first)

There would be 3 significant figures, (7, 8 and zero)

b. examine and describe cells