MathDB

2

Part of 2019 AMC 10

Problems(2)

Friendly Factorials

Source: 2019 AMC 10A #2

2/8/2019
What is the hundreds digit of (20!−15!)?(20!-15!)?
<spanclass=′latex−bold′>(A)</span>0<spanclass=′latex−bold′>(B)</span>1<spanclass=′latex−bold′>(C)</span>2<spanclass=′latex−bold′>(D)</span>4<spanclass=′latex−bold′>(E)</span>5<span class='latex-bold'>(A) </span> 0 \qquad<span class='latex-bold'>(B) </span> 1 \qquad<span class='latex-bold'>(C) </span> 2 \qquad<span class='latex-bold'>(D) </span> 4 \qquad<span class='latex-bold'>(E) </span> 5
AMCAMC 10AMC 10 Afactorial2019 AMC 10A2019 AMC
Counterexample

Source: 2019 AMC 10B #2 / 12B #2

2/14/2019
Consider the statement, "If nn is not prime, then n−2n-2 is prime." Which of the following values of nn is a counterexample to this statement?
<spanclass=′latex−bold′>(A)</span>11<spanclass=′latex−bold′>(B)</span>15<spanclass=′latex−bold′>(C)</span>19<spanclass=′latex−bold′>(D)</span>21<spanclass=′latex−bold′>(E)</span>27<span class='latex-bold'>(A) </span> 11 \qquad <span class='latex-bold'>(B) </span> 15 \qquad <span class='latex-bold'>(C) </span> 19 \qquad <span class='latex-bold'>(D) </span> 21 \qquad <span class='latex-bold'>(E) </span> 27
AMCAMC 10AMC 10 B2019 AMC 10BprimesAMC 12 B