Arithmetic

Ignore the 2nd q, he says the smallest 2 prime no.s > 50, so I thought it's 2 53s, but he wants 53+59= 112

I didn't know we should memorize beyond 50 for prime no.s

Question 1 is essentially asking how many odd numbers there are between 0 and 10: 5.

Question 2 is testing whether you understand what prime numbers are. You don't need to memorize them, but you should know pretty quickly that any even number > 2 = not prime, and adding any two numbers together = multiple of 3's = not prime. Multiple of 4's follow the same rule as 2's. Multiple of 5's are numbers that end in 0 or 5. Multiple of 6's follow the same rule as 3's. 7 is harder, but GMAT won't ask you to figure out a whether a large non-obvious number is prime.

The key for #2 is to memorize the methods of establishing whether a number is multiples of numbers between 2 to 9.

Can you explain what's meant by remainder of 1? I don't get it

nor this "adding any two numbers together = multiple of 3's = not prime"

Remainder: https://en.wikipedia.org/wiki/Remainder

So any odd number divided by 2 will have 1 as the remainder.

24: 2 + 4 = 6. 6 is a multiple of 3, and therefore 24 is a multiple of 3.

126: 1 + 2 + 6 = 9. 9 is a multiple of 3, and therefore 126 is a multiple of 3.

3257757 = 3 + 2 + 5 + 7 + 7 + 5 + 7 = 36. 36 is a multiple of 3, and so forth...

I strongly recommend you to pick a MGMAT Number Property book, and go through it very very thoroughly.

Perfect! Thanks for explaining, especially the multiple part.

Btw these qs are from GRE not GMAT, and this was the 1st q on an actual CAT. So you might notice they are way easier than the GMAT.