Question

if a child pours 210ml juice in a glass and then drinks one seventh of it, after which he adds another 30% of what is left in the glass to it, how much juice is in the glass

Answer 1

Initially: 210 mL. Drinks 1/7 of it, what left is

$$V_1=210·(1-1/7)=180\text{ mL}.$$

Then the child adds 30% of what is left in the glass to the 180 mL:

$$V_2=180(1+30/100)=234\text{ mL}.$$