B+b+b = 12kg
t+b = 5kg
j-t = 99kg
j+b+t = ?

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Step-by-step explanation:

B + B + B = 12kg

3B = 12kg             divide both sides by 3

B = 4kg

Substitute to T + B = 5kg:

T + 4kg = 5kg               subtract 4kg from both sides

T = 1kg

Substitute to J - T = 99kg:

J - 1 = 99kg                add 1 to both sides

J = 100kg

Therefore:

J + B + T = 4kg + 1kg + 100kg = 105kg