Working Capital

Hello Sir,
Hope you are keeping well and safe!

I was working on the ratio’s and in Inventory turnover period we calculate it by Avg Inventory divided by COS , so in BPP Text book , question starts with :
20X3 20X2
$m $m
Sales revenue 2,065.0 1,788.7
Cost of sales 1,478.6 1,304.0
Gross profit 586.4 484.7
Current assets
Inventories 119.0 109.0

For average inventory should we calculate it by (119+109)/2 , they have considered only closing inventory, can you please further explain why ?

Thanks in adance!