Greetings from Brasil...
Let's do
Fe = iron
Z = zinc
(remembering the periodic table)
According to the question of the statement:
Fe = 18
and
"... intake of iron is 18 This is 4 less than twice the recommended daily intake of zinc..."
That is, 18 of Fe is double that of zinc minus 4 units
Writing this situation mathematically:
Fe = 2 · Z - 4
18 = 2Z - 4
18 + 4 = 2Z
22 = 2Z
22/2 = Z
