Since P = V x I, a 10% reduction of power would lead to a 10% reduction in the product of voltage and current. What is left is 90% of the original power:
.9P = .9(V x I).
If we assume that current must be the same, then we can regroup the terms on the right-hand side as follows:
.9P = (.9V) x I
In this case, voltage is also reduced by 10% (100% - 90% = 10%).