(b) 1.03 x 10^24 atoms Mn into grams
(c) 3.60 x 10^22 atoms He
into grams
(d) 2.88 x 10^15 atoms N
into grams
(e) 2.88 x 10^15 atoms U into grams

a.  94 g of Mn

b. 0.239 g of He

c. 6.69ₓ10⁻⁸ g of N

d. 1.14ₓ10⁻⁶ g of U

Explanation:

This is the way to make the conversions:

Number of atoms → Moles of element → grams

We begin from the information that: 1 mol of atoms has 6.02×10²³ (NA) atoms

Let's convert all the numbers to amount of moles (number of atoms / NA)

a.  1.03 x 10²⁴ of Mn / 6.02×10²³ = 1.71 moles of Mn

b. 3.60 x 10²² of He / 6.02×10²³ = 0.0598 moles of He

c. 2.88 x 10¹⁵ of N /6.02×10²³ = 4.78ₓ10⁻⁹ moles of N

d. 2.88 x 10¹⁵ of U /6.02×10²³ = 4.78ₓ10⁻⁹ moles of U

We convert the moles to mass (moles . molar mass)

a.  1.71 moles of Mn . 54.94 g /1mol = 94 g of Mn

b. 0.0598 moles of He . 4 g /1mol = 0.239 g of He

c. 4.78ₓ10⁻⁹ moles of N . 14 g /1mol = 6.69ₓ10⁻⁸ g of N

d. 4.78ₓ10⁻⁹ moles of U . 238.03 g /1mol = 1.14ₓ10⁻⁶ g of U

c-14 is carbon-14, or radiocarbon.   it is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons.