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The hard part of this question seems (to me at least) to be how to translate
the English into equivalent math.

"1/4 more monkeys than zebras" means
- 25% more monkeys than zebras
- there are 5u monkeys if there are 4u zebras
- ratio of monkeys : zebra = 5:4

"1/4 less monkeys than zebras" means
- 25% less monkeys than zebras
- there are 3z monkeys if there are 4z zebras
- ratio of monkeys : zebra = 3:4

Once we are clear about that, this problem is quite straightforward:

SOLUTION 1:
           MONKEYS    ZEBRAS
PAST         5u         4u
NOW         5u-16      4u+16

So 5u-16 : 4u+16 = 3 : 4. This means:
  20u-64 = 12u+48
      8u = 112
       u = 14
There were 4x14 + 16 = 72 zebras in the end.

SOLUTION 2:
There is another way which requires less algebra.

        MONKEYS : ZEBRAS : TOTAL
PAST       5    :   4    :   9
NOW        3    :   4    :   7

Since the 16 monkeys that died were replace by 16 zebras,
we know that the TOTAL did not change. So we can scale
up the ratios to make TOTAL the same:

            MONKEYS : ZEBRAS : TOTAL
PAST x 7u     35u   :  28u   :  63u
NOW  x 9u     27u   :  36u   :  63u

So we see that there is a decrease of 8u in monkeys and
an increase of 8u in zebras. So 8u = 16 and u = 2.
There were 36u = 36 x 2 = 72 zebras in the end.

@Kiki4684: Sorry for the late replies. I see that you posted
many questions on Friday. Unfortunately, due to a technical
glitch in the forum, I was unable to post any reply. It may have
happened to some of the others who regularly answer questions too.
In any case, the glitch was resolved and now I can post again.
Hope the answers are useful for you to understand the topics.


 

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