how to find the date that corresponds to a specified sum

let's have 2 rows of data:

date	1/5/2017	1/15/2017	1/20/2017	2/15/2017	2/25/2017	3/5/2017
items received	5	10	5	5	10	10

how create formula tells me @ date sum of items received equals 20? 1/20/2017.

see screenshot

formula in b3 is =sum($b$2:b2) ; copied along row

formula in b4 =index(b1:g1,1,match(20,b3:g3,0))

i used cond. formatting highlight 20

best wishes

Office / Excel / Windows other

Cororate