Since 4x/5 is the reciprocal of 1/20, that means that 4x/5 = 20/1, or 20. Using this info, we can form the equation [tex] \frac{4x}{5}=20 [/tex] . From there we can solve for x.
Firstly, multiply both sides by 5: [tex] 4x=100 [/tex]
Next, divide both sides by 4 and your answer will be [tex] x=25 [/tex]