We want to check which number of rows cannot form a decreasing-by-3 sequence whose sum is 630 .A decreasing-by- 3 sequence for n rows looks like:a,(a−3),(a−6),…,(a−3(n−1))Sum of n rows:S=2n​[2a−3(n−1)]=630We test each option to see if it gives a positive integer value of a.Check n=323​(2a−6)=6302a−6=420⇒2a=426⇒a=213✓ Valid (integer)Check n=42(2a−9)=6302a−9=315⇒2a=324⇒a=162✓ Valid (integer)Check n=525​(2a−12)=6302a−12=252⇒2a=264⇒a=132✓ Valid (integer)Check n=63(2a−15)=6302a−15=210⇒2a=225⇒a=112.5x Not an integer → impossible