1/3 of an hour = 20 minutes
8 hours = 480 minutes
So to find the number of periods we divide 480 by 20. Answer is 24
40,000
All you do is multiply 4,000 by 10 to find out what 10 times of your number is, if 4,000 is only 1/10 of it.
So... let's say the amounts invested were "a" at 6% and "b" at 7.5%.
ok.. hmm what's 6% of a? well, (6/100) * a or 0.06a.
what's 7.5% of b? well, (7.5/100) * b or 0.075b.
now... we know, whatever "a" and "b" are, they total the investment of 5000 bucks, thus
a + b = 5000and the interest yielded was 337.50, thus
0.06a + 0.075b = 337.50thus

solve for "a", to see how much was invested at 6%.
what about "b"? well, b = 5000 - a.