What is the retail price of a shirt, if the wholesale price is $12 and the price increase is 150%?

Get a ruler in your hands. Measure things until you start to understand how a ruler works. Measure some stuff and figure out where the center is. Say you measure a book and it's 7/8" thick. You look at your ruler and see that every eighth is divided into two sixteenths, so obviously half of 7/8" is going to be 7/16". If you write that out you have 1/2 x 7/8 = 7/16. And you notice that 1/2 is divided into 2/4 and then into 4/8 and so on, so you can convert anything to anything by multiplying all the numbers on top and then all the numbers on bottom.

Other rulers are divided into 10 and 100 parts. But an inch is still an inch, so anything on one ruler can be translated to the other ruler. A half inch on one ruler is 5/10 or 50/100 on the other. An eighth inch is just 12.5 marks when you have 100 marks per inch. A metric ruler divides an inch into 25.4 parts, so a half inch would be 12.7 of those parts. Pretty simple, isn't it? Practice this a bit and people will think you went to wizard school.

Percent is simply a ruler with 100 marks. The only confusion is trying to keep track of what the marks represent. So when you say "150%" you have to say percent of what. Usually the sale price is expressed as wholesale cost plus markup. If the markup is 50% then the sale price is 150% of wholesale. Sometimes numbers are expressed as a percent of the sale price, but 150% would not make sense in that case. So we have to assume that the question refers to percent of wholesale, even though it is improperly stated.

$12 x 150% = $12 x 1.5 = $18

Using the ruler analogy, 100 marks are worth $12 so each mark is worth $0.12 and 150 marks are worth $0.12 x 150 = $18