book Maths Puzzle Numerical Ability

a friend has trouble simplifing 20-(-38).Explain how to solve this problem

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book Other Question

How many gallons of 40% acid solution must be mixed with 30 gallons of 16% solution to obtain solution of 30% acid? Here is my problem:

Find two consecutive numbers whose sum is 30 and whose product is 225.

I can figure out the answer (15 and 15) but can't figure out how to put
it into a