Cost of a watermelon is 4 times of an orange. So an orange costs ¼ of a watermelon.

8 ÷ 4 = 2

So 8 oranges cost the same as 2 watermelons.

So cost of 4 watermelons and 8 oranges

= cost of 4 watermelons and 2 watermelons

= cost of 6 watermelons

So 1/5 of John's money is the same as the cost of 6 watermelons.

1 - 1/5 = 4/5

He had 4/5 of his total money remaining.

4/5 x ½ = 4/10 = 2/5

He used 2/5 of his total money to buy more watermelons.


2/5 is twice of 1/5. So extra number of watermelons he bought
= 2 x 6
= 12

Total = 4 + 12 = 16 watermelons

unit cost
o: 1 u
w: 4 u

cost
o: 8 u
w: 16 u
total cost = 24 u

1/5 = 24 u

remaining money = 4/5
so
1/2 of remaining money
= 1/2 x 4/5 = 2/5


1/5 = 24 u
2/5 = 48 u

total no. of watermelons
= 4 + 48/4 = 16


...