First, create an equation with the criteria given.
If we let x be your number,

(

is twice the product of 3 and x, and if it exceeds

by 10, then we add 10 to

to make the two sides of the equation equal.)
Now, solve for

.
Find the circumference which is 2*3.14*13 and then usethe central angle formula
To find the answer, you will need to divide the total number of balloons by the number of balloons in a pack.
275 ÷ 2 = 137.5
Because you can't buy half of a package, Celia should probably buy 138 packages.