Let Ram's cost price = Rs. 100

Ram's selling price =\( \Large Rs. 100 + \frac{100 \times 20}{100} \)
= Rs. 120

Rahim's cost price = Rs. 120

Rahim's selling price = Rs. 120 + \( \Large \frac{120 \times 25}{100} \)
= Rs. 150
Robert's cost price = Rs. 150

If Robert's cost price is Rs. 150, then Ram's cost price = Rs. 100
Hence if Robert's cost price is Rs. 900, then
Ram's cost price = Rs. \( \Large\frac{100}{150} \times 900 = Rs. 600 \)