Aclown needed 275 balloons for a party he was going to, but the balloons only came in packs of 2.how many packs of balloons does he need to buy?

For this case what you need to know is that each package contains 2 balloons.
 Therefore, you can solve the problem by making the following rule of three:
 1 package ---> 2 balloons
 x > 275 balloons
 Clearing x we have:
 x = (275/2) * (1)
 x = 137.5
 Nearest whole number:
 x = 138
 
 I need to buy 138 packs of balloons