Hi,

Ask yourself

What fraction is 390 grams of 800 grams?

The answer is $\large \frac{390}{800}.$ Express this as a decimal and then to convert is to a percentage, multiply by 100.

As a partial check of you answer suppose the question was

What percentage is 400 grams of 800 grams?

400 is half of 800 so the answer would be 50%.

390 is slightly less than 400 and so is your answer slightly less than 50%?

Penny