First row contains 7 logs. And logs are incrementing by 1 for each successive row. And number of rows is 20.
So using the A.P formula tn = a + (n-1)d we know
a=7, d=1 and n = 20 and we have to find t20, ie number of logs in 20th row.
t20 = 7 + (20 - 1)d
t20 = 7 +19*1
t20 = 26.
a) So there are 26 logs in the bottom row.
Total number of logs is given by the formula Sn = n/2 * [2a + (n-1)]d
So, S20 = 20/2 * [2*7 + (20-1)*1]
S20 = 10 * [ 14+19]
S20 = 10 * 33 = 330
b) So there are total of 330 logs.