Step-by-step explanation:
d=days not used
50-2d  =  40-1.5d
subract 40 each side
10-2d =1.5d
10= 0.5d
20=d
After 20 days the value of the gift  cards will be equal.
20*2=40      50-40=10 ($10)
20*1.5=30     40-30=10 ($10)
each card will have a value of $10  after 20 days,when both cards have equal value