For sake of convenience, reverse the series, then it would be:
21,19,17.....7
a=21, d = -2,
s(4) = 4/2 (21+15)
s(4)=2(36)
s(4) = 72
184.2 is the answer to 4 significant figure
1.5 meters
There is 30 cm in a foot and 5 feet times 30 is 150. There is 100 cm in a meter so 150/100 is 1.5 meters.