First find how much water and salt are in the original Solution:
since 5% =0.05, there are .05 * 80 lbs = 4.0 lbs of Salt, and 76 lbs water.
with 76 lbs water, a 24% solution would have a total of 0.24 * 76lbs = 18.24 lbs of salt.
Since there already is 4 lbs of salt in there, you need to add 14.24 lbs of salt
Hope that helps!