Your maths is ntialala like comrades...that annual worthness!
Seems this student is hard headed and out to frustrate the teacher. That one year is factored in my maths above. You can see 900k for 5 years while 792k for 4 years?? In first year A makes more than B but as years progress the gap reduces and overturns.Just take a pen and paper and do it.

, teachers have to be patient and understanding - that said some teachers tend to get biased in their teaching methods and focus on what is straight forward leaving aside the juicy parts.

If one needs 900k in a loan, using your example when will the sacco individual get the 900k of disposable cash for use.

now we need a way of calculating the cost of the time difference and of course the opportunity cost - we are assuming people take out loans for a particular need, otherwise if we are to look at your example then its better off stashing the 20k per month under a mattress for 5 years.

The best way to compare the two is to hold all other factors constant i.e. assume you can take 1M loan from either of the two

Then put them side by side

SACCO will beat bank there

@nakujua B will have to contribute 22k for 14 months = 308k. He now qualifies for that 900k and invests it for 46 months(3.83 yrs). A invests all the 900k for 60 months (5yrs). Both at 20% profit p.a cmpnd.**When B qualifies for the loan ;Here A will be commiting 22,855/- on loan only while B Commits 23700 on loan and 2k on shares total 25700 mothly. If we ensure all put the 25700 into use for our study,then A will have a surplus of 2845/- for the remaining 46 months to invest and we assume it earns 8% p.a
Calculations:

Loans payments:
A. 22,855 x 60 months = 1,371,300.
B. 23,700 x 48 months = 1,137,600.
Interest paid
A. (1371-900)k = 471k
B. (1137-900)k = 237k

ROI on loan invested.
A 900k x (1.2)^5 =2240k
B 900k x (1.2)^3.83 = 1810k.

Surplus/Money at Sacco without dvdnds.
A.( 2845 x 46 ) = 130k
B. 308+(2 x 46) = 400k
The above with 8% dvdnd p.a compounded
A. = 158k
B. = 570k

Net gains by both = Earnings - expenses (intrest on loan):
A ( 2240+158) - 471 = 1927k
B ( 1810+570) - 237 = 2143k.
Make your own inferences.

you are complicating the calculation when you bring in investments. Lets focus on the cost of getting and paying the loan for now, since investments bring in more parameters.

Loan amount 900k
A gets in 0 months (60 months repayment)
B gets in 13 months (47 months repayment)

___________________A: __________ B:
monthly payment __ 22,854 ______ 24,175
total payment ____ 1,371,245 ___ 1,131,379
total interest ___ 471,245 _____ 231,379

At this point we consider the direct benefits at the end of 5 years;
____________ A: _____________ B:
Surplus ____ 62,087 (diff) __ 300,000 (shares)
Interest ___ -471,245 ________ -231,379
Time _______ 13 months ______ 0 months

A = -409167 + (cost of 13 months)
B = -231,379 + (cost of disposing of 300k shares)

and that is my hesabu jua kali style