If X, Y, Z and W are in proportion. X = 3, Y = 13 and W = 39, determine Z.

5 20 15 9

9

Fourth proportion of A, B, C = $\frac{B\;\times\;C}{A}$ Z = \(\frac{X\; \times\; W}{Y}\)    = \(\frac{3\; \times\; 39}{13}\)       = 9