**Calculation of MR**

To calculate MR, we need to use the following formula:

MR = ∆TR / ∆Q

Where,
∆TR = Change in total revenue
∆Q = Change in quantity demanded

**Calculation of the initial revenue**

Initial revenue can be calculated by using the following formula:

Initial revenue = Price x Quantity demanded

Here, the initial price is 20 and the initial quantity demanded is 10 units.

Initial revenue = 20 x 10 = 200

**Calculation of the new price**

The new price is decreased by 5% from the initial price.

New price = Initial price - (5% of initial price)
New price = 20 - (0.05 x 20) = 19

**Calculation of the new quantity demanded**

The new quantity demanded is increased by 10% from the initial quantity demanded.

New quantity demanded = Initial quantity demanded + (10% of initial quantity demanded)
New quantity demanded = 10 + (0.1 x 10) = 11

**Calculation of the new revenue**

The new revenue can be calculated by using the same formula as the initial revenue.

New revenue = New price x New quantity demanded
New revenue = 19 x 11 = 209

**Calculation of MR**

Now, we can use the formula for MR to calculate the marginal revenue.

∆TR = New revenue - Initial revenue
∆TR = 209 - 200 = 9

∆Q = New quantity demanded - Initial quantity demanded
∆Q = 11 - 10 = 1

MR = ∆TR / ∆Q
MR = 9 / 1 = 9

Therefore, the marginal revenue is 9.

**Explanation**

The calculation of MR is based on the change in revenue and quantity demanded due to a change in price. In this scenario, the price decreased by 5% and the quantity demanded increased by 10%. As a result, the new revenue increased by 4.5% (209-200/200) and the marginal revenue is positive. This means that decreasing the price by 5% has led to an increase in revenue and that the company is earning more profit by selling more units at a lower price.