Let us convert the percentages to decimal format first.. so 5% is just 5/100 or 0.05 and 15% is just 15/100 or 0.15
so hmmm, so, let's say it needs "x" amount and "y" amount of each respectively, so, whatever "x" and "y" are, they must add up to 100, and whatever their concentration is, must add up to what the mixture yields
thus

solve for "x"
what's "y"? well, y = 100 - x
Answer:
y = 10 when x is -4
will be the correct answer
We know that 1 lb = 16 oz
So lets convert the 20 lb to oz and then compare the two quantities in consistent units.
If 1 lb = 16 oz then 20 lb are equal to 20*16 oz, that is
20 lb = 20*16 oz = 320 oz
therefore,
320 oz are greater than 272 oz, or,
20 lb are greater that 272 oz
There is not an answer to an expression.
-3+p is an expression, unless there was an equal sign and a sum at the end, that is when we can solve for the variable x.
-3+p simply equals -3+p