2nd question, you need to supply a daily intake for each heifer, lbs per day.
You calculate DM available providing a utilization percentage as you did, then divide that by (number of heifers X intake per heifer per day)

Third question, the amount of gain is the difference between .85 and .65 of mature weight, divided by gestation length.

7. Use the following prices ($ per ton as is) to rank the following feedstuffs from question 4 from cheapest to most expensive source of CP:
#4 Corn Silage---$ 20
How to calculate: 2000x.34 (DM%)= 680 lbs DM/ton 680dm lbs/ton x .05 = 34 lb protein/ton
Cost: $20/34 lb protein/ton = .588 = 59 cents.
Do the same for all feedstuffs relative to their DM%

8. How much of the Bermudgrass hay from question 4 would a 1200-lb cow, 3-4 months postpartum, need to eat to meet her TDN requirements each day? Can she eat that much hay? If not, choose another feedstuff and balance a ration for her TDN requirement.
TDN= digestible crude protein + digestible crude fiber + digestible nitrogen-free extract + digestible crude fat x 2.25
13.9lb TDN/ 56% TDN Bermuda grass hay/89% DM= 27.88 Bermuda hay as fed.
2.1/.1= 21/.89 = 23.6lb/day to meet TDN and CP.
She can eat that much hay/day.
Dry matter intake – 26 Lb/day
Crude Protein – 2.1 Lb/day
TDN – 13.9Lb/DAY

18. Assume 2,000 lb DM per acre,
60% utilization,
10 lb DM intake per lb of gain,
500 lb steers at $140 per cwt and
600 lb steers at $120 per cwt,
90 days time available. Calculate how many steers you could put on one acre of rented pasture. Profit states:WY, CO Renting loss states:WI, IA
Carrying capacity- annual forage production x seasonal utilization rate/ ADGx grazing season length
Generally, there should be 1acre per 2 cows.
2000*.60/90x.03 (% body weight to gain) = 444.44.
Allowing only one Steer to be raised on this 1 acre of land.

To get you started, 60% of 2000 lb DM per acre is 1200 lb actually consumed. At 10:1 F:G, (10 lb of feed per pound of gain) that much DM consumed will provide 120 lb of gain per acre. The difference betwen 600 lb and 500 lb is 100 lb of gain per steer, which is the stipulated gain over 90 days in the problem 120/100 = 1.2 steers per acre. the increase in value per steer is $20 (the difference between the value of a 600 lb steer and a 500 lb steer), 1.2 X $20 - $24 per acre, the breakeven value/max rent per acres

Can a 1,200 – lb beef cow with a newborn calf eat enough of a 5% CP, 50% TDN hay to meet her protein and energy requirements? Show calculations to support your answer
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The type/source of the hay and the CP content of the hay are irrelevant, you've been given TDN composition.
You need to provide 1) a DM intake capacity for the cow, and 2) a daily TDN requirement. From the lecture, we could use 2 to 3 % of BW as intake, and 14 lb TDN as a requirement (in the homework, it was 26 lb DM intake 13.8 kg of TDN). We divide her TDN need by the TDN in the hay, 14/.5, and get 28 lb of DMI. In other words, the cow would have to eat 28 lb of that hay to obtain 14 kg of TDN. 28 lb is within the 2 to 3% of BW, so one correct answer is 'yes, she can' . If you stipulate DM intake at 2% of BW, and say, "No, she can't", that would be a correct answer as well, based on the information you provided.

Calculate the breeding weight and average daily gain during gestation for a first-calf heifer that will weigh 1500 lb at maturity. (Hint: we recommend breeding heifers at 65% of mature weight and calving at 85% of mature...i did: 1500*.65= 975lb-450lb = 525lb/180 days = 2.9 lb gain/day

Accurately calculate the target weights and weight gains for replacement heifers.
In this example, we are feeding a group of Angus heifers averaging 500 pounds at weaning in a herd where mature cows average 1,200 pounds at body condition score 5 or 6. The heifers are weaned on November 1 and we want to begin breeding on June 1. We have decided that the target for this herd will be 65% of mature weight by June 1 (approximately 63% if May 1).
Target Weight Calculation Example
Target Weight by June 1 = 1200 lb x .65 = 780 lb
Total weight gain needed = 780 - 500 = 280 lb Days to June 1 = 211
Average daily gain needed to reach Target Weight = 280 lb ¸ 211 days = 1.32 lb/day

7) Accurately calculate the target weights and weight gains for replacement heifers.
Example: Expected mature cow weight 1,000 lb X 0.65
Weight desired at breeding= 650 lb - Present weaning weight -450-lbs
Gain needed=  200-lbs (divide by the number of days until breeding season)
Days until breeding season 280
Gain needed 1.1-lb per day