Payback period, discounted payback, irr...?

2 mutually exclusive investments:
A had a cost of 500K and will provide 200k in savings in 7 years.
B had a cost of 750K and will provide 300K for 6 years.
what is the payback period for each project. if required rate of return is 10%, what is the discounted payback for each project. what is the irr of each project?

-Payback Period
A) Cost is $500,000
7 years = $200,000
$500,000/$200,000 = 2.5X
2.5X * 7years =17.5years

B) Cost is $750,000
6 years = $300,000
$750,000/$300,000 = 2.5X
2.5X * 6years = 15years

*Assumed cash flows can be annual and will provide cash inflows for foreseeable future.

-Discounted Payback Period - Same concept as above, except you discount each cash inflow at 10% (I assume that scenario A is actually $28,571.43/yr and scenario B is $50,000/yr). **Will take longer to recover investment**

-IRR - This is just the rate at which NPV = 0. Very easy with a financial calculator.
A) -18.77 (since there are only known cf of 7 years)
B) -21.22 (since there are only known cf of 6 years)


note the different assumptions for each question