Explanation:
the ratio is 25/50 (25 kg in 50 minutes).
as a ratio is basically a fraction (just with a special meaning), we can simplify it as a "normal" fraction :
25/50 = 1/2
in other words, the hippo can eat 1 kg in 2 minutes.
now the answer to the question is easy :
how many minutes to eat 60 kg ?
remember, when we multiply one part of the fraction by a factor, we need to multiply the other post by the same factor to keep the overall value.
so, 60 kg = 60×1 kg
we multiply the numerator by 60, therefore, we need to multiply the denominator by 60 too. and we get
1/2 × 60/60 = 60/120
it takes the hippo 120 minutes to eat 60 kg.