TR4 billion is the correct answer as calculated below
If R1 billion represent the remaining 25%
"R1=25\\%"
Then, an increase in consumption will be calculated as follows;
"=\\frac {R1\\times 75\\%}{25\\%}"
"=R3 Billion"
Hence, an increase in national income will as much as;
"R3+R1=R4 \\space Billion"
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