You look for a pattern. The first must be 210, because 0, 1 and 2 are the first three digits, so reversing them gives us 210. Find another: 321, but we can also have 320 and 310. Another: 432, but we can also have 430, 431, 420, 421, 410. Another: 543, also 542, 541, 540, 532, 531, 530, 521, 520, 510. As the whole numbers get larger there is more room for the tens and units. So a pattern has been established.
In all we have 1 (210)+3 (321, 320, 310)+6 (432, 431, 430, 421, 420, 410)+10 (543, 542, 541, 540, 532, 531, 530, 521, 520, 510)=20. The pattern is 1+(1+2)+(1+2+3)+(1+2+3+4).