Answer:
7+21+15+19+.....+79
Here 21 be replaced by 11.
Now., common diffrence btw terms , d = 11-7 = 4
first term , a = 7
Last term , l = 79
number of terms = ( a + l ) ÷ d
=( 7 + 79 ) ÷ 4 = 86 ÷ 4 = 21.5 { It cannot be in decimal }
There's some mistake in the terms you provided please check your question again...
