$A$ and $B$ are two heavy steel blocks. If $B$ is placed on top of $A$, the weight increases by $60\%$. How much weight will reduce with respect to the total weight of $A$ and $B$, if $B$ is removed from the top of $A$?

Let the weight of block $A$ be $W_A=100\text{units}$.
When $B$ is placed on top of $A$, the weight increases by $60\%$. This increase is due to the weight of block $B$.
Therefore, the weight of block $B$ is:

$$W_B=60\%\text{ of }{W}_A=\frac{60}{100}\times 100=60\text{units}$$

The total initial weight of both blocks together is:

$$W_{\text{total}}=W_A+W_B=100+60=160\text{units}$$

When $B$ is removed, the weight reduces by the weight of $B$, which is $60\text{units}$.
We need to find this reduction as a percentage of the total weight of $A$ and $B$:

$$\text{Percentage reduction}=\left(\frac{W_B}{W_{\text{total}}}\right)\times 100\%$$

$$\text{Percentage reduction}=\left(\frac{60}{160}\right)\times 100\%=\frac{3}{8}\times 100\%=37.5\%$$