20 imperial gallons = 90.9218 litres
so i'd say you'd need to buy 91 bottles or...
20 US Liquid gallons = 75.7082 litres
so you'd have to buy 76 bottles.
Hope this answers your question! It depends what gallon you're working from.
The sum of the first n odd numbers is n squared! So, the short answer is that the sum of the first 70 odd numbers is 70 squared, i.e. 4900.
Allow me to prove the result: odd numbers come in the form 2n-1, because 2n is always even, and the number immediately before an even number is always odd.
So, if we sum the first N odd numbers, we have

The first sum is the sum of all integers from 1 to N, which is N(N+1)/2. We want twice this sum, so we have

The second sum is simply the sum of N ones:

So, the final result is

which ends the proof.
I guess this is what you mean 3√a^10 and 3√b^7
Answer: $45
Explanation: $7 multiplied by 6 people = 42. Then you add the addition of the $3 launch fee to make the raft go