In this question they've given us the formula and they just want us to plug in the numbers and get the value.
P is the original value, P=$600
r is the depreciation rate, r=10%=0.1
t is the time in years, t=2

Answer: $486
An easy way is to do it, if you get confused by multiplying fractions, is by turning the fractions into decimals. Since you are trying to find N you want to multiply the 1/5 by the 1/3.
1/5 = 0.2
1/3 = 0.33333333333333333333333333333333
if you multiply those you get 0.06666666666666666666666666666667 which when put into a fraction it equals 1/15
but if you don't want to do that just try to remember that you want to get by itself. and when multiplying fractions in this situation the bottom numbers only matter.. so 5 x 3 = 15 1/15
would you like me to put it in a document to show you?
Let the amount invested at 4% be = x
Let the amount invested at 3% be = y
Given is:
or
.... (1)
As, total income for the two investments is $194, so equation is:
....(2)
Putting value of x from (1) in (2)




And x=5200-y

Hence, money invested at 4% is $3800 and money invested at 3% is $1400
The answer is b I hope this helps