Odd $n$: origin 2015, $t=-3..3$, $\sum t^2=28$; $a=146.9/7=20.99$, $b=-10.8/28=-0.386$; $\hat y=20.99-0.386t$; 2020 ($t=5$): $19.06$. OR: 5-yr moving averages listed.

Main part — linear trend for birth rate (7 years).

Origin at the middle year 2015, $t = \text{year} - 2015$, so $t = -3,-2,-1,0,1,2,3$; $\sum t = 0$, $\sum t^2 = 28$.

Because $\sum t = 0$:

$a = \frac{\sum y}{n} = \frac{146.9}{7} = 20.9857, \qquad b = \frac{\sum ty}{\sum t^2} = \frac{-10.8}{28} = -0.3857$

Trend line: $\hat{y} = 20.99 - 0.3857\,t$ (origin 2015).

For 2020, $t = 2020 - 2015 = 5$:

$\hat{y} = 20.9857 - 0.3857(5) = 20.9857 - 1.9286 = 19.06$

OR — five-yearly moving averages of production (tons).