Answer:
d + 85
Step-by-step explanation:
Look at it like this,
suppose you have 1 DVD, and the mother buys 85 DVDs then you have 1 + 85 = 86 DVDS
or when you have 2 DVDs, then you have 2 + 85 = 87 DVDs afterwards.
notice that you always take the number of DVDs you already have, and add 85 to it.
d is just a number, but you just don't know it yet. you can change d by any number, and you know d+85 is the number of DVDs afterwards
