we know that in 1990 his weight was 25% more than in 1980, so if we take 1980's weight to be "x", thus "x" is then 100%, move forward 10 years later and his weight is "k", which is 25% more, or 100% + 25% = 125%.
now, if "k" is 125%, what is "x" anyway?
[tex]\bf \begin{array}{ccll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
k&125\\
x&100
\end{array}\implies \cfrac{k}{x}=\cfrac{125}{100}\implies \cfrac{100k}{125}=x\implies \cfrac{4}{5}k=x[/tex]
