Answer:
a)
10 markers : 7 USD
24 markers : 14 USD
42 markers : 21 USD
b)
64 markers : 28 USD
Explanation:
10 markers : 7 USD
24 markers : 14 USD
For 10 markers, the price is 7 USD;
For 14 additional markers, the price increased by 7 USD;
Following the pattern, an additional 18 markers would see the price further increase by 7 USD;
So:
24 + 18 = 42
42 markers : 21 USD
Then, an additional 22 markers would cost 7 USD more than this;
42 + 22 = 64
64 markers : 28 USD