One number is 5 times another. the sum of their reciprocals is 2/5. determine the smaller number.

So x=5y
reciprocal of x=1/x then

sum of reciprocals means 1/x+1/y=2/5
subsitute x=5y for x in 1/x+1/y=2/5 and get
1/5y+1/y=2/5
multiply 1/y by 5/5 to make it 5/5y so you can add since the denomenators (bottom number) are the same

1/5y+5/5y=2/5
6/5y=2/5
multiply both sides by 5y
6=2y
divide both sides by 2
3=y
put this into x=5y so 
x=5(3)
x=15
y=3

The answer would be: b