Maths problem: how to do b, c, d. Thanks?

May: marked price = $400 × (1+25%) = $500

July:
   selling price (?) = $400 × (1-10%) = $360
   marked price = $360 ÷ (1-40%) = $600

Percentage increasing in the marked prices from
May to July:
    ($600-$500)/$500 × 100% = 20%