I have no idea where to start???

30

Step-by-step explanation:

Cross Multiply - 15 times 42 = 630

630 = 21t

What is 630 / 21... = 30

t = 30

21/15=42/t

Rewrite the equation as 42/t=21/15.

42/t=21/15

Cancel the common factor of 21 and 15.

42/t=7/5

Multiply the numerator of the first fraction by the denominator of the second fraction. Set this equal to the product of the denominator of the first fraction and the numerator of the second fraction.

42⋅5=t⋅7

Solve the equation for t.

t=30

Step-by-step explanation:

Hope it is helpful