Help with 5-8 with breakdown please!!

5)Corp sells a marble slab for $1,000. Fixed costs are $30,000, variable costs are $400 per slab. What is the margin of safety in dollars assuming budgeted sales are 75 slabs? 6)Corp sells a single product. 8,000 units were sold resulting in $80,000 of sales revenue, $20,000 of variable costs, and $10,000 of fixed costs. If variable costs decrease by $1 per unit, the new margin of safety (based on actual sales) is how much (in dollars)? 7)Corp is holding a fundraising banquet. He has two options for the banquet: OPTION one: a. Fixed rental cost of $1,000 b. $12 per person for food OPTION two: a. Fixed rental cost of $3,000 b. A caterer who charges $8.00 per person for food