How much chromium-51 will be left after 7 months?

It seems there's more information in you question that you haven't posted.

7 months = 7 * (365/12) = 213 days

The half-life of chromium 51 is 27.7 days.

After 7 months, chromium 51 will have undergone 7.69 half-lives.

To solve for the ending amount we'll use the formula:

Ending Amount = Beginning Amount / 2^n where 'n' = # of half-lives

We'll say the beginning amount =1

Ending Amount = 1 / 2^7.69

Ending Amount = 0.0048426082

Basicaly if you started with 1 gram of chromium 51, after 7 months you would have 0.0048426082 grams.

Source: https://www.1728.org/halflife.htm

Step-by-step explanation: