1, four clothes only cost three clothes, so the formula is: 3 divided by 4=0.75=75%, which is 75% off.

2. The discount for the Little Scientist series is: 60*0.7=42 yuan.

The discount for natural science series is: 40*0.6=24 yuan.

So the two books were originally 100 yuan, but now they are only 42+24=66 yuan.

Formula: 66 divided by 100=0.66=66% means 66% discount.

3. 150 divided by 0.6=250 yuan.