x is inversely proportional to y; y = 7 when x = 5; find x when y = 90.

Use the product constancy principle: $x_1 \times y_1 = x_2 \times y_2$. Here, $5 \times 7 = x_2 \times 90$, so $x_2 = \frac{35}{90} = \frac{7}{18}$.

Confusing 'inversely proportional' with 'directly proportional'. If it were directly proportional, one would use $\frac{x_1}{y_1} = \frac{x_2}{y_2}$, but here $x \times y$ must remain constant.

This concept of inverse variation is fundamental in physics, particularly in Ohm's Law ($I = V/R$) or Boyle's Law ($PV = k$), where two variables maintain a constant product.