As mentioned in the problem upper and lower threshold voltage is 1.5V which is not possible. Hence consider UTP as +1.5V and LTP as -1.5V.

Design :

Since UTP = LTP

Step 1 : Determine hysteresis voltage

$$V_H = V_{UTP} - V_{LTP}$$ Given, UTP = 1.5V and LTP = -1.5V $$V_H = 1.5 – (- 1.5) = 3$$

Step 2 : Determine R1

$$V_H = \frac{2R_2}{R_1 + R_2} × V_{SAT}$$ Assume $V_{SAT}$=13V and $R_2$ = 10KΩ $$3 = \frac{2 × 10K}{10K + R_1} × 13V$$ $$\boxed{R_1 = 76.66KΩ}$$

Step 3 : Designed Circuit Diagram