Identical coins were laid out in the form of an equally-sided triangle so that each side

Identical coins were laid out in the form of an equally-sided triangle so that each side of the triangle consists of 20 coins, how many coins are used in total

An equilateral triangle is one in which all sides are equal. Means:
20×3 = 60
But we must not forget that each side has one common coin, which means you need to subtract 3 coins from the result
20×3 – 3 = 57 coins
60×3-3 = 177 coins
90×3-3 = 267 coins
105×3-3 = 312 coins
210×3 -3 = 627 coins