can someone please explain how to solve these problems? thanks

12. A solenoid, 49 cm long and 2.8 cm in diameter, is to produce a magnetic field 1000 times stronger than the Earth's magnetic field (0.58 x 10-4 T) at its centre. If the maximum current is 3.9 A, how many turns must the solenoid have? (a) 2200 (b) 4600 (c) 5800 (d) 9300 (Electromagnetic Induction, Giancoli Chapter 21) 13. A fixed 10.8 cm-diameter wire coil is perpendicular to a magnetic field 0.48 T pointing up. In 0.16 s the field is changed to 0.25 T pointing down. What is the average induced EMF in the coil. (a) 4.2 x 102 V (b) 4.2 x 10' V (c) 4.2 x 10-1 V (d) 4.2 x 10-2 V 14. A circular loop in the plane of the paper lies in a 0.85 T magnetic field pointing into the paper. The loop's diameter changes from 20.0 cm to 8.0 cm in 0.50s. What is the magnitude of the induced EMF? (a) 15 mV (b) 37 mV (c) 45 mV (d) 58 mV 15. Following from the previous question, what is the direction and the average induced current if the coil resistance is 2.5 0? (a) 18 mA in the clockwise direction. (b) 28 mA in the anti-clockwise direction. (c) 15 mA in the clockwise direction. (d) 35 mA in the anti-clockwise direction