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

Ththe weight of Lior in 1990 in terms of k is

Let the weight of Lior in 1980 be "w"

If Lior's weight increased by 25%, his new weight was "k". This can be expressed as:

w + (25% of w) = k

w + (0.255w) = k

w + 0.15w = k

1.25w = k

Divide both sides by 1.25

This shows that the weight of Lior in 1990 in terms of k is

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An increase of 25% is represented by the multiplier 1.00 + 0.25, or 1.25.
Then Lior's 1980 weight, mult. by 1.25, results in his 1990 weight, or k kg.

Let his 1980 weight be w.  Then 1.25w = k.  Divide both sides by 1.25 to obtain w, Lior's 1980 weight:

1.25 w = k
    = (4/5)k, or 4k/5 (answer)
  1.25      1.25

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