— SteemCN

OK, so here is my reasoning behind my answer:

Because 3 horses make 30, then each horse equal 10.
Since 1 horse equals 10 and that 1 horse + 2 pairs of horseshoes make 18, then 2 pairs of horseshoes make 8 and that 1 pair of horseshoes equal 4.
Now that 1 pair of horseshoes equal 4 and that 1 pair of horseshoes - 1 pair of boots make 2, then 1 pair of boots equal 2.
In the last line, because there is only ONE boot instead of a pair, then the value of 1 boot must be half of the value of a pair of boots, since 1 boot is half of a pair of boots. Likewise, because there is only ONE horseshoe instead of a pair, then the value of 1 boot must be half of the value of a pair of horseshoes.
Therefore, 1 boot equals 2 and 1 horseshoe equals 1.
As a result, the final equation is 1 + 10 * 2.
According to the order of operations, one multiplies before adding, so one must multiply the 10 and 2 before adding the one, which is how I got 21.

Sorry this post was so long. Thank you to anyone who actually took the time to read the whole thing. Please correct me if my reasoning is wrong.