The twins ordered a cheeseburger each so 2*13.50 = $<<2*13.50=27.00>>27.00 for both cheeseburgers
Everyone ordered a dessert so 4*6 = $<<4*6=24.00>>24.00 for dessert
When you add all the meals together they spent 25.50+35.00+27.00+8.50+24.00 = $<<25.50+35.00+27.00+8.50+24.00=120.00>>120.00
Zach's tip was 20% so .20*120.00 = $<<20*.01*120=24.00>>24.00
When you add up the meals $120.00 and Zach's tip of $24.00 then the total cost is 120.00 + 24.00 = $<<120+24.00=144.00>>144.00
#### 144