1. **State the problem:** We are given that after a 20% price decrease, the GST on a computer is 150. We need to find the GST on the computer before the price decrease. 2. **Define variables:** Let the original price of the computer be $P$. 3. **Price after 20% decrease:** The new price after a 20% decrease is $$P_{new} = P - 0.20P = 0.80P$$ 4. **GST relationship:** GST is proportional to the price. Let the GST rate be $r$. Then, $$\text{GST after decrease} = r \times P_{new} = r \times 0.80P = 150$$ 5. **Find GST before decrease:** The GST before the decrease is $$\text{GST before} = r \times P$$ 6. **Express $r$ from step 4:** $$r = \frac{150}{0.80P}$$ 7. **Substitute $r$ into GST before:** $$\text{GST before} = \frac{150}{0.80P} \times P = \frac{150}{0.80}$$ 8. **Calculate:** $$\text{GST before} = \frac{150}{0.80} = 187.5$$ **Final answer:** The GST on the computer before the price decrease was $187.5$.