Last year, Ann had $10,000 to invest. She invested some of it in an account that paid 7% simple interest per year, and she invested the rest in an account that paid 8% simple interest per year. After one year, she received a total of $790 in interest. How much did she invest in each account

Amount invested at 7% = $1,000

Amount invested at 8% = $9,000

Explanation

Let the amount invested

at 7% = X

at 8% = Y

According to given condition

X + Y = $10,000 ( Eq 1)

and

0.07X + 0.08Y = $790 ( Eq 2)

By multiplying ( Eq 1) with 0.07

0.07X + 0.07Y = $700 ( Eq 3)

By subtracting ( Eq 3) from ( Eq 2)

0.07X - 0.07X + 0.08Y - 0.07Y = $790 - $700

0.08Y - 0.07Y = $790 - $700

0.01Y = $90

Y = $90 / 0.01

Y = $9,000

X + $9,000 = $10,000

X = $10,000 - $9,000

X = $1,000

Check:

$1,000 x 7% + $9,000 x 8% = $790

$790 = $790

answer: marginal utility of the fourth brownie is 0.

explanation: marginal utility is the addtional utility gained from the consumption of one more unit of a good. it is given by,

so, marginal utility at the 4th unit will be given by

since, the values are not given here, you can simply find marginal utility at fourth unit by subtracting total utility at 3rd unit from total utility at 4th unit.