From 1980 to 1990, lior’s weight increased by 25%. if his weight was k kilograms in 1990, what was it in 1980?

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?

The percent increase is 25%