One way to do this would be to take 118 and multiply by 1.3. Don't do that. What a pain that would be. A better method would be to ask yourself, "what's 30% of 118?" That's not super easy, but 10% of 118 is, right? Ten percent of 118 is 11.8. Now multiply that in your head by 3. Pretty easy if you treat the 11 and .8 separately. 11x3 is 33. Now just remember that for a second while you do .8x3, which is 2.4, and then add 33 and 2.4 to get 35.4. Then add that to 118 to get the answer.
This looks scary but it's not too bad if you know this trick. A good method for adding sloppy numbers is to just borrow a few from one number and give it to the other to make one of them not sloppy. I'd borrow 2 from 35.4 and give it to 118, making them 33.4 and 120. These are considerably easier to add, and we get 153.4.
Doing arithmetic in your head seems scary but it's not hard if you practice and it's much preferable to the calculator, which is so unwieldy it's basically a disadvantage rather than an advantage.
BONUS METHOD: I showed the arithmetic earlier to illustrate that it can be done easily in your head. But the following method is even easier and faster. If you ever have to multiply, add, or subtract two numbers to get the answer, consider just looking at the last digit and comparing it to the answer choices.
In this case, we just need to multiply 1.3 by 118. It's not easy to do in one step, but we do know that whatever the number is, its last digit should be .4. That's because 3x8 equals 24, which ends in 4. If I multiplied 34895466846293 and 658938, whatever the answer is would end in a 4 as well. When we see the answer choices, we notice that none of them end in 4, but two of them end in .40, and that's because they're given in cents rather than dollars. So they really do end in .4. Hmmm. We have two candidates. However, the store marked the lamp up, not down, so the price needs to be a bit higher than 118, not 35.4 dollars, eliminating A. Thus, it's D.
BONUS BONUS METHOD: 118 is almost 120. 30% is almost 1/3. So since 1/3 of 120 is 40, we know 30% of 118 should be around 40. When estimating it's always a good idea to know whether you're overestimating or underestimating. We know that 40 is an overestimate, since we grew both numbers. So if we add 40 to 118 to get 158, we know the real answer should be just a tad less than that. So it's D again.
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