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4

Step-by-step explanation:

3(a+2) = 26-2a

3a+6=26-2a

3a+2a=26-6

5a=20

a=20/5

a=4

The element is remaining after 7 minutes = 60.23 grams

Step-by-step explanation:

The equation for mass decay :

, where = initial mass, A = mass after t minutes.

Given:

r = 26.3% = 0.263

t = 7 minutes

Hence, the element is remaining after 7 minutes = 60.23 grams