Question 1137780
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You say T = 4rB^2 "is not it".<br>
That's because that answer is a formula for calculating T from r and B; but the problem didn't ask for a formula.  It asked for the new value of T given new values of r and B.<br>
You can get the answer by plugging the new values in the formula.<br>
However, since the problem only asks for the value of T with new values of r and B, there is no need to find a general formula.  You can get the answer just by using the stated joint variation to see how the given value of T changes when the values of r and B change.<br>
The given value of r is 7; the new value of r is 10.  Since T varies directly as r, the new value of r multiplies the given value of T by 10/7.<br>
The given value of B is 6; the new value of B is 12.  Since T varies directly as the square of B, the new value of B multiplies the given value of T by (12/6)^2 = 2^2 = 4.<br>
So the new value of T is<br>
{{{(1008)(10/7)(4) = 144*10*4 = 5760}}}