Please explain with math, not Excel!

The demand for subassembly S is 120 units in week 7. Each unit of S requires 1 unit of T and 1 unit of U. Each unit of T requires 2 units of V, 2 units of W, and 1 unit of X. Finally, each unit of U requires 2 units of Y and 3 units of Z. One firm manufactures all items. It takes 2 weeks to make S, 1 week to make T, 2 weeks to make U, 2 weeks to make V, 3 weeks to make W, 1 week to make X, 2 weeks to make Y, and 1 week to make Z. The product structure. The time-phased product structure. Level 0 S X =