Calculate the cost of units in the first tariff block.

The initial meter reading is $5.3 \text{ kWh}$. The first tariff block is $0-100 \text{ kWh}$ at $\text{R}2.775755/\text{kWh}$.
The remaining units in the first block that can be purchased are:

The cost for these $94.7 \text{ kWh}$ is:

Lebogang purchased $\text{R}398.00$ worth of electricity. The money remaining after the first block purchase is:

The tariff for the second block ($101-400 \text{ kWh}$) is $\text{R}3.248405/\text{kWh}$.
The units purchased with the remaining money are:

Total kWh purchased = Units from first block + Units from second block

The new meter reading is the initial reading plus the total kWh purchased:

The calculated meter reading is $141.63 \text{ kWh}$. Rounded to the nearest whole number, this is $142 \text{ kWh}$.
The meter reading after purchase shown in Table 3 is $142.4 \text{ kWh}$. Rounded to the nearest whole number, this is also $142 \text{ kWh}$.
Since both values round to the same whole number, the meter reading after purchase is indeed correct.