35kg - a mass of sugar syrup
30% of 35kg it's a mass of sugar
30% = 30 : 100 = 0.3
30% of 35kg = 0.3 · 35kg = 10.5kg - a mass of sugar
Add xkg water. Then we have (35 + x)kg of sugar syrup and still 10.5kg of sugar.
10.5kg is 25% of (35 + x)kg .
(35 + x) -- 100%
10.5 -- 25% multiply both sides by 4
42 -- 100%
Therefore we have the equation:
35 + x = 42 subtract 35 from both sides
x = 7
