The problem involves calculating electricity costs based on different tariff blocks and meter readings. Could you please provide a detailed solution with steps for each calculation?

Block 1 is "1 - 100 KWh" with a cost of "209,70c" per KWh.

Since 1 Rand = 100 cents, divide the cents value by 100.

The tariff for block 1 in rands is $\boxed{\text{R2,0970}}$.

Previous Reading (28/02/2023) = 23 901 KWh
Current Reading (31/03/2023) = 24 501 KWh

The value of D, the amount in kilowatt (KWh) used in March 2023, is $\boxed{\text{600 KWh}}$.

Total amount paid = 101 277.5 cents

• Block 1 (1 - 100 KWh): 100 KWh at 209,70c/KWh (R2,0970/KWh) Cost for Block 1 = $100 \text{ KWh} \times \text{R}2,0970/\text{KWh} = \text{R}209,70$ Remaining amount = R1012,775 - R209,70 = R803,075

• Block 2 (101 - 400 KWh): This block covers 300 KWh (400 - 100). Tariff is 245,41c/KWh (R2,4541/KWh). Cost for Block 2 = $300 \text{ KWh} \times \text{R}2,4541/\text{KWh} = \text{R}736,23$ Remaining amount = R803,075 - R736,23 = R66,845

• Block 3 (401 - 650 KWh): This block covers 250 KWh (650 - 400). Tariff is 267,38c/KWh (R2,6738/KWh). Since the remaining amount (R66,845) is less than the full cost of Block 3 (R2,6738/KWh $\times$ 250 KWh = R668,45), only a portion of Block 3 was used.

KWh used in Block 3 = $\frac{\text{Remaining amount}}{\text{Tariff for Block 3}}$

Total KWh used = KWh from Block 1 + KWh from Block 2 + KWh from Block 3
Total KWh used = $100 \text{ KWh} + 300 \text{ KWh} + 25 \text{ KWh}$
Total KWh used = $425 \text{ KWh}$
The number of KWh they used for that period is $\boxed{\text{425 KWh}}$.